Unrolled build for #145335

Rollup merge of #145335 - clarfonthey:wtf8-core-alloc, r=Mark-Simulacrum

Move WTF-8 code from std into core and alloc

This is basically a small portion of #129411 with a smaller scope. It *does not*\* affect any public APIs; this code is still internal to the standard library. It just moves the WTF-8 code into `core` and `alloc` so it can be accessed by `no_std` crates like `backtrace`.

> \* The only public API this affects is by adding a `Debug` implementation to `std::os::windows::ffi::EncodeWide`, which was not present before. This is due to the fact that `core` requires `Debug` implementations for all types, but `std` does not (yet) require this. Even though this was ultimately changed to be a wrapper over the original type, not a re-export, I decided to keep the `Debug` implementation so it remains useful.

Like we do with ordinary strings, the tests are still located entirely in `alloc`, rather than splitting them into `core` and `alloc`.

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Reviewer note: for ease of review, this is split into three commits:

1. Moving the original files into their new "locations"
2. Actually modifying the code to compile.
3. Removing aesthetic changes that were made so that the diff for commit 2 was readable.

You can review commits 1 and 3 to verify these claims, but commit 2 contains the majority of the changes you should care about.

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API changes: `impl Debug for std::os::windows::ffi::EncodeWide`

Author: rust-timer

library/alloc/src/lib.rs

@@ -159,6 +159,7 @@
 #![feature(unicode_internals)]
 #![feature(unsize)]
 #![feature(unwrap_infallible)]
+#![feature(wtf8_internals)]
 // tidy-alphabetical-end
 //
 // Language features:
@@ -232,6 +233,8 @@ pub mod sync;
 #[cfg(all(not(no_global_oom_handling), not(no_rc), not(no_sync)))]
 pub mod task;
 pub mod vec;
+#[cfg(all(not(no_rc), not(no_sync), not(no_global_oom_handling)))]
+pub mod wtf8;
 
 #[doc(hidden)]
 #[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]