Use arrayvec 0.7, drop smallvec 0.6

[workingjubilee]

With the arrival of min const generics, many alt-vec libraries have
updated to use it in some way and arrayvec is no exception. Use the
latest with minor refactoring.

Also, rustc_workspace_hack is the only user of smallvec 0.6 in the
entire tree, so drop it.

[rust-highfive]

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

[klensy]

With arrayvec updates there was some issues with performance, so i guess it's better to do a perf run?

[Mark-Simulacrum]

@bors try @rust-timer queue

I wouldn't expect any difference, but we can definitely check.

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit e8eb691 with merge 8663640101c59d348cf2968c0d02ff47966bcc6e...

[bors]

☀️ Try build successful - checks-actions
Build commit: 8663640101c59d348cf2968c0d02ff47966bcc6e (8663640101c59d348cf2968c0d02ff47966bcc6e)

[rust-timer]

Queued 8663640101c59d348cf2968c0d02ff47966bcc6e with parent 88b99de, future comparison URL.

[rust-timer]

Finished benchmarking try commit (8663640101c59d348cf2968c0d02ff47966bcc6e): 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
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

[workingjubilee]

Looks like some potential gains and some potential very small losses but mostly noise? I'm not the most experienced here at reading these tea leaves, though.

[Mark-Simulacrum]

Seems overall fine.

@bors r+

[bors]

📌 Commit e8eb691 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit e8eb691 with merge 7f4afdf...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 7f4afdf to master...