Use DroplessArena where we know the type doesn't need drop

[bugadani]

This PR uses a single DroplessArena in resolve instead of three separate TypedArenas.

DroplessArena checks that the type indeed doesn't need drop, so in case the types change, this will result in visible failures.

[rust-highfive]

r? @varkor

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[lcnr]

might influence perf

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit 9b453d6 with merge 5650bcda4c0ae36febd02d8202432fdf7d187c96...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 5650bcda4c0ae36febd02d8202432fdf7d187c96 (5650bcda4c0ae36febd02d8202432fdf7d187c96)

[rust-timer]

Queued 5650bcda4c0ae36febd02d8202432fdf7d187c96 with parent f965120, future comparison URL.

[rust-timer]

Finished benchmarking try commit (5650bcda4c0ae36febd02d8202432fdf7d187c96): 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

[lcnr]

looks like a small perf improvement.gj

r? @lcnr

@bors r+

[bors]

📌 Commit 9b453d6 has been approved by lcnr

[bors]

⌛ Testing commit 9b453d6 with merge 6b9fbf2...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: lcnr
Pushing 6b9fbf2 to master...