Implement sound map value aliasing

This implementation gets rid of the unsound `ShallowCopy` trait (#74 &
rust-lang/unsafe-code-guidelines#35 (comment)),
and replaces it with a wrapper type around aliased values.

The core mechanism is that the wrapper type holds a `MaybeUninit<T>`,
and aliases it by doing a `ptr::read` of the whole `MaybeUninit<T>` to
alias. It then takes care to only give out `&T`s, which is allowed to
alias.

To drop, the implementation casts between two different generic
arguments of the new `Aliased` type, where one type causes the
`Aliased<T>` to drop the inner `T` and the other does not. The
documentation goes into more detail. The resulting cast _should_ be safe
(unlike the old `ManuallyDrop` cast).

The removal of `ShallowCopy` makes some of the API nicer by removing
trait bounds, and obviates the need for `evmap-derive`. While I was
going through, I also took the liberty of tidying up the external API of
`evmap` a bit.

The implementation passes all tests, and I _think_ it is sound (if you
think it's not, please let me know!).

Note that this does _not_ take care of #78.

Fixes #74.
Fixes #55 since `ShallowCopy` is no longer neeeded.
Also fixes #72 for the same reason.

Author: jonhoo

Cargo.toml

@@ -1,2 +1,2 @@
 [workspace]
-members = ["evmap", "evmap-derive", "left-right"]
+members = ["evmap", "left-right"]

azure-pipelines.yml

@@ -12,18 +12,6 @@ stages:
         minrust: 1.40.0
         codecov_token: $(CODECOV_TOKEN_SECRET)
         dir: "evmap"
-    - job: derive
-      displayName: "Test evmap-derive"
-      pool:
-        vmImage: ubuntu-latest
-      steps:
-        - template: install-rust.yml@templates
-          parameters:
-            components:
-              - rust-src
-        - script: cargo test
-          displayName: cargo test
-          workingDirectory: "evmap-derive"
     - job: benchmark
       displayName: "Check that benchmark compiles"
       pool:

evmap-derive/Cargo.toml

@@ -25,7 +25,6 @@ eviction = ["indexed", "rand"]
 indexmap = { version = "1.1.0", optional = true }
 smallvec = "1.0.0"
 hashbag = "0.1.2"
-bytes = { version = "0.5", optional = true }
 rand = { version = "0.7", default-features = false, features = ["alloc"], optional = true }
 left-right = { version = "0.9.0", path = "../left-right" }