Auto merge of #136932 - m-ou-se:fmt-width-precision-u16, r=scottmcm

Reduce formatting `width` and `precision` to 16 bits

This is part of https://github.com/rust-lang/rust/issues/99012

This is reduces the `width` and `precision` fields in format strings to 16 bits. They are currently full `usize`s, but it's a bit nonsensical that we need to support the case where someone wants to pad their value to eighteen quintillion spaces and/or have eighteen quintillion digits of precision.

By reducing these fields to 16 bit, we can reduce `FormattingOptions` to 64 bits (see https://github.com/rust-lang/rust/pull/136974) and improve the in memory representation of `format_args!()`. (See additional context below.)

This also fixes a bug where the width or precision is silently truncated when cross-compiling to a target with a smaller `usize`. By reducing the width and precision fields to the minimum guaranteed size of `usize`, 16 bits, this bug is eliminated.

This is a breaking change, but affects almost no existing code.

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Details of this change:

There are three ways to set a width or precision today:

1. Directly a formatting string, e.g. `println!("{a:1234}")`
2. Indirectly in a formatting string, e.g. `println!("{a:width$}", width=1234)`
3. Through the unstable `FormattingOptions::width` method.

This PR:

- Adds a compiler error for 1. (`println!("{a:9999999}")` no longer compiles and gives a clear error.)
- Adds a runtime check for 2. (`println!("{a:width$}, width=9999999)` will panic.)
- Changes the signatures of the (unstable) `FormattingOptions::[get_]width` methods to use a `u16` instead.

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Additional context for improving `FormattingOptions` and `fmt::Arguments`:

All the formatting flags and options are currently:

- The `+` flag (1 bit)
- The `-` flag (1 bit)
- The `#` flag (1 bit)
- The `0` flag (1 bit)
- The `x?` flag (1 bit)
- The `X?` flag (1 bit)
- The alignment (2 bits)
- The fill character (21 bits)
- Whether a width is specified (1 bit)
- Whether a precision is specified (1 bit)
- If used, the width (a full usize)
- If used, the precision (a full usize)

Everything except the last two can simply fit in a `u32` (those add up to 31 bits in total).

If we can accept a max width and precision of u16::MAX, we can make a `FormattingOptions` that is exactly 64 bits in size; the same size as a thin reference on most platforms.

If, additionally, we also limit the number of formatting arguments, we can also reduce the size of `fmt::Arguments` (that is, of a `format_args!()` expression).

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