Auto merge of rust-lang#117697 - WaffleLapkin:non-null-convenience-ops, r=Amanieu

Non null convenience ops

Based on rust-lang/libs-team#251.

I went through all of the methods on `*mut` and added every method, which does not require additional safety conditions, to `NonNull`. (exceptions: `guaranteed_eq`, `guaranteed_ne`, `with_metadata_of`, it's unclear if they are useful here...)

I'm also not sure what types should the "second pointer parameter" be. `*mut`/`*const` might be more permissible, but given that `NonNull` doesn't coerce to them, it might also be annoying. For now I chose the "use `NonNull` everywhere" path, but I'm not sure it's the correct one...

<sub>I'm eepy, so I probably messed up somewhere while copying...</sub>

cc `@scottmcm`
r? libs-api

Author: bors

library/core/src/lib.rs

@@ -178,6 +178,7 @@
 #![feature(is_ascii_octdigit)]
 #![feature(isqrt)]
 #![feature(maybe_uninit_uninit_array)]
+#![feature(non_null_convenience)]
 #![feature(offset_of)]
 #![feature(offset_of_enum)]
 #![feature(ptr_alignment_type)]