Auto merge of #81156 - DrMeepster:read_buf, r=joshtriplett

Implement most of RFC 2930, providing the ReadBuf abstraction

This replaces the `Initializer` abstraction for permitting reading into uninitialized buffers, closing #42788.

This leaves several APIs described in the RFC out of scope for the initial implementation:

* read_buf_vectored
* `ReadBufs`

Closes #42788, by removing the relevant APIs.

library/std/src/lib.rs

@@ -304,6 +304,7 @@
 #![feature(maybe_uninit_extra)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
+#![feature(maybe_uninit_write_slice)]
 #![feature(min_specialization)]
 #![feature(mixed_integer_ops)]
 #![feature(must_not_suspend)]
@@ -320,6 +321,7 @@
 #![feature(pin_static_ref)]
 #![feature(portable_simd)]
 #![feature(prelude_import)]
+#![feature(ptr_as_uninit)]
 #![feature(ptr_internals)]
 #![feature(rustc_attrs)]
 #![feature(rustc_private)]