Rollup merge of #135073 - joshtriplett:bstr, r=BurntSushi

Implement `ByteStr` and `ByteString` types

Approved ACP: https://github.com/rust-lang/libs-team/issues/502
Tracking issue: https://github.com/rust-lang/rust/issues/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).`

r? `@BurntSushi`

library/core/src/lib.rs

@@ -111,6 +111,8 @@
 #![feature(array_ptr_get)]
 #![feature(asm_experimental_arch)]
 #![feature(bigint_helper_methods)]
+#![feature(bstr)]
+#![feature(bstr_internals)]
 #![feature(const_carrying_mul_add)]
 #![feature(const_eval_select)]
 #![feature(core_intrinsics)]
@@ -336,6 +338,8 @@ pub mod ascii;
 pub mod asserting;
 #[unstable(feature = "async_iterator", issue = "79024")]
 pub mod async_iter;
+#[unstable(feature = "bstr", issue = "134915")]
+pub mod bstr;
 pub mod cell;
 pub mod char;
 pub mod ffi;