Rollup merge of rust-lang#80720 - steffahn:prettify_prelude_imports, r=camelid,jyn514

Make documentation of which items the prelude exports more readable.

I recently figured out that rustdoc allows link inside of inline code blocks as long as they’re delimited with `<code> </code>` instead of `` ` ` ``. I think this applies nicely in the listing of prelude exports [in the docs](https://doc.rust-lang.org/std/prelude/index.html). There, currently unformatted `::` and `{ , }` is used in order to mimick import syntax while attatching links to individual identifiers.

## Rendered Comparison
### Currently (light)
![Screenshot_20210105_155801](https://user-images.githubusercontent.com/3986214/103661510-1a87be80-4f6f-11eb-8360-1dfb23f732e8.png)

### After this PR (light)
![Screenshot_20210105_155811](https://user-images.githubusercontent.com/3986214/103661533-1f4c7280-4f6f-11eb-89d4-874793937824.png)

### Currently (dark)
![Screenshot_20210105_155824](https://user-images.githubusercontent.com/3986214/103661571-2a9f9e00-4f6f-11eb-95f9-e291b5570b41.png)

### After this PR (dark)
![Screenshot_20210105_155836](https://user-images.githubusercontent.com/3986214/103661592-2ffce880-4f6f-11eb-977a-82afcb07d331.png)

### Currently (ayu)
![Screenshot_20210105_155917](https://user-images.githubusercontent.com/3986214/103661619-39865080-4f6f-11eb-9ca1-9045a107cddd.png)

### After this PR (ayu)
![Screenshot_20210105_155923](https://user-images.githubusercontent.com/3986214/103661652-3db26e00-4f6f-11eb-82b7-378e38f0c41f.png)

_Edit:_ I just noticed, the “current” screenshots are from stable, so there are a few more differences in the pictures than the ones from just this PR.

Author: Dylan-DPC

std/src/prelude/mod.rs

@@ -28,53 +28,53 @@
 //! The current version of the prelude (version 1) lives in
 //! [`std::prelude::v1`], and re-exports the following:
 //!
-//! * [`std::marker`]::{[`Copy`], [`Send`], [`Sized`], [`Sync`], [`Unpin`]}:
+//! * <code>[std::marker]::{[Copy], [Send], [Sized], [Sync], [Unpin]}</code>,
 //!   marker traits that indicate fundamental properties of types.
-//! * [`std::ops`]::{[`Drop`], [`Fn`], [`FnMut`], [`FnOnce`]}: various
+//! * <code>[std::ops]::{[Drop], [Fn], [FnMut], [FnOnce]}</code>, various
 //!   operations for both destructors and overloading `()`.
-//! * [`std::mem`]::[`drop`][`mem::drop`]: a convenience function for explicitly
+//! * <code>[std::mem]::[drop][mem::drop]</code>, a convenience function for explicitly
 //!   dropping a value.
-//! * [`std::boxed`]::[`Box`]: a way to allocate values on the heap.
-//! * [`std::borrow`]::[`ToOwned`]: the conversion trait that defines
+//! * <code>[std::boxed]::[Box]</code>, a way to allocate values on the heap.
+//! * <code>[std::borrow]::[ToOwned]</code>, the conversion trait that defines
 //!   [`to_owned`], the generic method for creating an owned type from a
 //!   borrowed type.
-//! * [`std::clone`]::[`Clone`]: the ubiquitous trait that defines
-//!   [`clone`][`Clone::clone`], the method for producing a copy of a value.
-//! * [`std::cmp`]::{[`PartialEq`], [`PartialOrd`], [`Eq`], [`Ord`]}: the
+//! * <code>[std::clone]::[Clone]</code>, the ubiquitous trait that defines
+//!   [`clone`][Clone::clone], the method for producing a copy of a value.
+//! * <code>[std::cmp]::{[PartialEq], [PartialOrd], [Eq], [Ord]}</code>, the
 //!   comparison traits, which implement the comparison operators and are often
 //!   seen in trait bounds.
-//! * [`std::convert`]::{[`AsRef`], [`AsMut`], [`Into`], [`From`]}: generic
+//! * <code>[std::convert]::{[AsRef], [AsMut], [Into], [From]}</code>, generic
 //!   conversions, used by savvy API authors to create overloaded methods.
-//! * [`std::default`]::[`Default`], types that have default values.
-//! * [`std::iter`]::{[`Iterator`], [`Extend`], [`IntoIterator`],
-//!   [`DoubleEndedIterator`], [`ExactSizeIterator`]}: iterators of various
+//! * <code>[std::default]::[Default]</code>, types that have default values.
+//! * <code>[std::iter]::{[Iterator], [Extend], [IntoIterator], [DoubleEndedIterator], [ExactSizeIterator]}</code>,
+//!   iterators of various
 //!   kinds.
-//! * [`std::option`]::[`Option`]::{[`self`][`Option`], [`Some`], [`None`]}, a
+//! * <code>[std::option]::[Option]::{[self][Option], [Some], [None]}</code>, a
 //!   type which expresses the presence or absence of a value. This type is so
 //!   commonly used, its variants are also exported.
-//! * [`std::result`]::[`Result`]::{[`self`][`Result`], [`Ok`], [`Err`]}: a type
+//! * <code>[std::result]::[Result]::{[self][Result], [Ok], [Err]}</code>, a type
 //!   for functions that may succeed or fail. Like [`Option`], its variants are
 //!   exported as well.
-//! * [`std::string`]::{[`String`], [`ToString`]}: heap-allocated strings.
-//! * [`std::vec`]::[`Vec`]: a growable, heap-allocated vector.
+//! * <code>[std::string]::{[String], [ToString]}</code>, heap-allocated strings.
+//! * <code>[std::vec]::[Vec]</code>, a growable, heap-allocated vector.
 //!
-//! [`mem::drop`]: crate::mem::drop
-//! [`std::borrow`]: crate::borrow
-//! [`std::boxed`]: crate::boxed
-//! [`std::clone`]: crate::clone
-//! [`std::cmp`]: crate::cmp
-//! [`std::convert`]: crate::convert
-//! [`std::default`]: crate::default
-//! [`std::iter`]: crate::iter
-//! [`std::marker`]: crate::marker
-//! [`std::mem`]: crate::mem
-//! [`std::ops`]: crate::ops
-//! [`std::option`]: crate::option
+//! [mem::drop]: crate::mem::drop
+//! [std::borrow]: crate::borrow
+//! [std::boxed]: crate::boxed
+//! [std::clone]: crate::clone
+//! [std::cmp]: crate::cmp
+//! [std::convert]: crate::convert
+//! [std::default]: crate::default
+//! [std::iter]: crate::iter
+//! [std::marker]: crate::marker
+//! [std::mem]: crate::mem
+//! [std::ops]: crate::ops
+//! [std::option]: crate::option
 //! [`std::prelude::v1`]: v1
-//! [`std::result`]: crate::result
-//! [`std::slice`]: crate::slice
-//! [`std::string`]: crate::string
-//! [`std::vec`]: mod@crate::vec
+//! [std::result]: crate::result
+//! [std::slice]: crate::slice
+//! [std::string]: crate::string
+//! [std::vec]: mod@crate::vec
 //! [`to_owned`]: crate::borrow::ToOwned::to_owned
 //! [book-closures]: ../../book/ch13-01-closures.html
 //! [book-dtor]: ../../book/ch15-03-drop.html