Make keyword and builtin sets static

[NoahTheDuke]

No need to initialize the keyword and builtin sets inline and then pass them around manually. Instead, define them as static (using the lazy_static library once_cell library) and reference them "globally".

[CAD97]

I'm positive on this change, but I'd prefer to use once_cell::Lazy to lazy_static.

[NoahTheDuke]

Sure! Never heard of that one but looks relatively similar so switching over should be easy enough.

[CAD97]

(it'd be interesting to make these maps a static phf map instead, but future work.)

I'd be happy to merge this with lints fixed.

[NoahTheDuke]

The change to once-cell was to align with std, right? I think it's now called SyncLazy, but it's still on nightly.

[CAD97]

Pretty much yeah, the once-cell API is on track to be std-available eventually.

[CAD97]

Merging #555 led to a merge conflict; r=me after that's fixed

[NoahTheDuke]

For some reason, running clippy locally doesn't show these errors so I think it's okay to remove the clippy::ptr_arg directives.

[NoahTheDuke]

Okay, figured it out. These warnings don't happen in the latest version of Rust + Clippy, but Pest uses 1.56.1, from last October. If this is something worth updating, I can do that.

[CAD97]

I bet I know what it is: the CI clippy is avoid-breaking-exported-api = false (predates that) but modern clippy won't suggest changing public API.

We definitely could use some restructuring of our CI pipeline to separate the MSRV checks from the clippy/fmt checks. See also #655.