Avoid `Layout::array()` in `raw_vec.rs`.

[nnethercote]

A non-generic equivalent results in less LLVM IR and faster compilation.

r? @Amanieu

[nnethercote]

This gave me wins of up to 1.9% locally, but changes to this code can be unpredictable in terms of performance, so let's see what it looks like on CI.

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit 894236f with merge f7180d1...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: f7180d1 (f7180d1a199036f6f96d6791cbe6f58b00a890a4)

[rust-timer]

Queued f7180d1 with parent 1b0ff9e, future comparison URL.

[rust-timer]

Finished benchmarking try commit (f7180d1): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[mati865]

Small max-rss improvement, minimal regression everywhere else.