Use const generics in SipHasher128's short_write

[Kobzol]

This was proposed by @michaelwoerister here.
A few comments:

1. I tried to pass &[u8; LEN] instead of [u8; LEN]. Locally, it resulted in small icount regressions (about 0.5 %). When passing by value, there were no regressions (and no improvements).
2. I wonder if we should use to_ne_bytes() in SipHasher128 to keep it generic and only use to_le_bytes() in StableHasher. However, currently SipHasher128 is only used in StableHasher and the short_write method was private, so I couldn't use it directly from StableHasher. Using to_le() in the StableHasher was breaking this abstraction boundary before slightly.

debug_assert!(LEN <= 8);

This could be done at compile time, but actually I think that now we can remove this assert altogether.

r? @the8472

[the8472]

debug_assert!(LEN <= 8);

This could be done at compile time, but actually I think that now we can remove this assert altogether.

Well, in principle this is covered by the spill capacity check but it's more explicit that way.

It's hot code and might impact compile time even though I don't really expect it to, so no rollup.
@bors r+ rollup=never

[bors]

📌 Commit 5fc2e56 has been approved by the8472

[bors]

⌛ Testing commit 5fc2e56 with merge 5c7c1a0f448ae46a5ef8becb90b398893372a851...

[bors]

💥 Test timed out

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

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[the8472]

Looks like #93329

@bors retry

[bors]

⌛ Testing commit 5fc2e56 with merge fc32303...

[bors]

☀️ Test successful - checks-actions
Approved by: the8472
Pushing fc32303 to master...

[rust-timer]

Finished benchmarking commit (fc32303): comparison url.

Summary: This benchmark run did not return any relevant results.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression