Auto merge of #118159 - EliasHolzmann:formatting_options, r=m-ou-se

Implementation of `fmt::FormattingOptions`

Tracking issue: #118117

Public API:
```rust
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub struct FormattingOptions { … }
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum Sign {
    Plus,
    Minus
}
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum DebugAsHex {
    Lower,
    Upper
}

impl FormattingOptions {
    pub fn new() -> Self;
    pub fn sign(&mut self, sign: Option<Sign>) -> &mut Self;
    pub fn sign_aware_zero_pad(&mut self, sign_aware_zero_pad: bool) -> &mut Self;
    pub fn alternate(&mut self, alternate: bool) -> &mut Self;
    pub fn fill(&mut self, fill: char) -> &mut Self;
    pub fn align(&mut self, alignment: Option<Alignment>) -> &mut Self;
    pub fn width(&mut self, width: Option<usize>) -> &mut Self;
    pub fn precision(&mut self, precision: Option<usize>) -> &mut Self;
    pub fn debug_as_hex(&mut self, debug_as_hex: Option<DebugAsHex>) -> &mut Self;

    pub fn get_sign(&self) -> Option<Sign>;
    pub fn get_sign_aware_zero_pad(&self) -> bool;
    pub fn get_alternate(&self) -> bool;
    pub fn get_fill(&self) -> char;
    pub fn get_align(&self) -> Option<Alignment>;
    pub fn get_width(&self) -> Option<usize>;
    pub fn get_precision(&self) -> Option<usize>;
    pub fn get_debug_as_hex(&self) -> Option<DebugAsHex>;

    pub fn create_formatter<'a>(self, write: &'a mut (dyn Write + 'a)) -> Formatter<'a>;
    }

impl<'a> Formatter<'a> {
    pub fn new(write: &'a mut (dyn Write + 'a), options: FormattingOptions) -> Self;
    pub fn with_options<'b>(&'b mut self, options: FormattingOptions) -> Formatter<'b>;
    pub fn sign(&self) -> Option<Sign>;

    pub fn options(&self) -> FormattingOptions;
}
```

Relevant changes from the public API in the tracking issue (I'm leaving out some stuff I consider obvious mistakes, like missing `#[derive(..)]`s and `pub` specifiers):

- `enum DebugAsHex`/`FormattingOptions::debug_as_hex`/`FormattingOptions::get_debug_as_hex`: To support `{:x?}` as well as `{:X?}`. I had completely missed these options in the ACP. I'm open for any and all bikeshedding, not married to the name.
- `fill`/`get_fill` now takes/returns `char` instead of `Option<char>`. This simply mirrors what `Formatter::fill` returns (with default being `' '`).
- Changed `zero_pad`/`get_zero_pad` to `sign_aware_zero_pad`/`get_sign_aware_zero_pad`. This also mirrors `Formatter::sign_aware_zero_pad`. While I'm not a fan of this quite verbose name, I do believe that having the interface of `Formatter` and `FormattingOptions` be compatible is more important.
- For the same reason, renamed `alignment`/`get_alignment` to `aling`/`get_align`.
- Deviating from my initial idea, `Formatter::with_options` returns a `Formatter` which has the lifetime of the `self` reference as its generic lifetime parameter (in the original API spec, the generic lifetime of the returned `Formatter` was the generic lifetime used by `self` instead). Otherwise, one could construct two `Formatter`s that both mutably borrow the same underlying buffer, which would be unsound. This solution still has performance benefits over simply using `Formatter::new`, so I believe it is worthwhile to keep this method.

Author: bors