0000.0003.md

# Integrate `core::windows::transcoding` into `core::unicode`

## Explanation

Windows has its own functions for converting between UTF-8 and their native encoding (ostensibly
UTF-16, but they don't quite follow the spec apparently). We currently do have a way to call these
functions in `core::windows::transcoding`, but they really should be part of `core::unicode`
(defined only when we're on Windows, obviously).

Furthermore, because these functions were written before `core::strings`, they still use
`std::string` and `std::wstring`. We can very easily implement a `WindowsEncoding` in
`core::strings` that calls the Windows-appropriate functions and uses `::WCHAR` code units, and also
perform the transcoding on `ImmutableSlice`s / `MutableSlice`s instead of `std::string`s and
`std::wstring`s.

## Task

- Move everything from `core::windows::transcoding` into `core::unicode`
- Add appropriate `#ifdef` guards to ensure the Windows functionality is only declared on Windows
  (it wouldn't compile anywhere else anyway)
- Convert the functions to use `ImmutableSlice`s as inputs and either `MutableSlice`s or
  `DynamicArray`s as outputs (similar to the existing bulk transcoding functions)
- Speaking of the bulk transcoding functions, rename the Windows functions to correspond to the
  same naming scheme
- Add an encoding traits class to `core::strings` that allows them to be used with Windows strings