Avoid a Vec allocation

[harrisonkaiser]

This was found using clippy with -D clippy::perf. Since we know the size of the slice at compile time, we can avoid heap allocating a Vec.

[harrisonkaiser]

Interesting: ci / test (msrv) failed:

error: package `regex-automata v0.4.3` cannot be built because it requires rustc 1.65 or newer, while the currently active rustc version is 1.63.0

regex-automata must have bumped their msrv since the last PR.

[Keats]

Thanks!