Auto merge of #77476 - tgnottingham:buffered_siphasher128, r=nnethercote

perf: buffer SipHasher128

This is an attempt to improve Siphasher128 performance by buffering input. Although it reduces instruction count, I'm not confident the effect on wall times, or lack-thereof, is worth the change.

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Additional notes not reflected in source comments:

* Implementation choices were guided by a combination of results from rustc-perf and micro-benchmarks, mostly the former.
* ~~I tried a couple of different struct layouts that might be more cache friendly with no obvious effect.~~ Update: a particular struct layout was chosen, but it's not critical to performance. See comments in source and discussion below.
* I suspect that buffering would be important to a SIMD-accelerated algorithm, but from what I've read and my own tests, SipHash does not seem very amenable to SIMD acceleration, at least by SSE.

Author: bors

Diff for: compiler/rustc_data_structures/src/lib.rs

@@ -28,6 +28,7 @@
 #![feature(const_panic)]
 #![feature(min_const_generics)]
 #![feature(once_cell)]
+#![feature(maybe_uninit_uninit_array)]
 #![allow(rustc::default_hash_types)]
 
 #[macro_use]