Add unstable 128 bit SipHash version for internal use in the compiler.

[michaelwoerister]

Discussion in #41215 seems to show that a 128 bit SipHash is the best compromise between hash quality and performance. This PR adds an unstable, hidden 128 bit variant to the SipHash implementation in libstd for use solely within the compiler. A future PR would then switch StableHasher to use this implementation.

What do you say, @rust-lang/libs? Acceptable?

r? @alexcrichton

[sfackler]

Since it's not something we'd want to stabilize, I think I'd mildly prefer for it to live in a rustc-internal crate if possible.

[michaelwoerister]

Since it's not something we'd want to stabilize, I think I'd mildly prefer for it to live in a rustc-internal crate if possible.

The problem with that is that the implementations of the 64 bit version and the 128 bit version are 95% the same. It's just an additional finish method and slightly adapted initialization that are needed for the 128 bit version.

[alexcrichton]

I'd personally lean the same way as @sfackler in that we probably want to vendor this in rustc internals for now. The duplication is indeed unfortunate, but hopefully not too bad as it's roughly just one module I think, right?

[michaelwoerister]

OK, I'll do that then. 👍