Implement ByteStr and ByteString types

Approved ACP: rust-lang/libs-team#502
Tracking issue: rust-lang#134915

These types represent human-readable strings that are conventionally,
but not always, UTF-8. The `Debug` impl prints non-UTF-8 bytes using
escape sequences, and the `Display` impl uses the Unicode replacement
character.

This is a minimal implementation of these types and associated trait
impls. It does not add any helper methods to other types such as `[u8]`
or `Vec<u8>`.

I've omitted a few implementations of `AsRef`, `AsMut`, and `Borrow`,
when those would be the second implementation for a type (counting the
`T` impl), to avoid potential inference failures. We can attempt to add
more impls later in standalone commits, and run them through crater.

In addition to the `bstr` feature, I've added a `bstr_internals` feature
for APIs provided by `core` for use by `alloc` but not currently
intended for stabilization.

This API and its implementation are based *heavily* on the `bstr` crate
by Andrew Gallant (@BurntSushi).

Author: joshtriplett

library/alloc/src/bstr.rs

@@ -103,6 +103,8 @@
 #![feature(async_fn_traits)]
 #![feature(async_iterator)]
 #![feature(box_uninit_write)]
+#![feature(bstr)]
+#![feature(bstr_internals)]
 #![feature(clone_to_uninit)]
 #![feature(coerce_unsized)]
 #![feature(const_eval_select)]
@@ -226,6 +228,8 @@ mod boxed {
     pub use std::boxed::Box;
 }
 pub mod borrow;
+#[unstable(feature = "bstr", issue = "134915")]
+pub mod bstr;
 pub mod collections;
 #[cfg(all(not(no_rc), not(no_sync), not(no_global_oom_handling)))]
 pub mod ffi;