Add ability to gc node caches

[CAD97]

Originally implemented in sorbus here: CAD97/sorbus#46

Also exposes the NodeCache (closes #53) so that the gc is usable. While GreenNodeBuilder has had with_cache for a while, it's been impossible to actually share the cache, as there was no way to get a cache to share!

[kjeremy]

Are these HashMap just because of drain_filter?

[CAD97]

Yes (unfortunately).

A further optimization step in the pipeline will take further advantage of HashMap::raw_entry_mut, so the switch to using set-like maps is probably inevitable, though.

[simonvandel]

What is the effect on peak memory usage for analysis stats using rust analyzer?

[CAD97]

It's not going to have an immediate impact because r-a doesn't (IIUC) yet hold on to or reuse node caches.

Additionally, it will only potentially have an impact when holding onto a node cache after the deletion of a parse tree. We aren't really generating any garbage to clean up; it's only useful if the cache outlives the tree(s) that it's built.

[BenjaminBrienen]

is this still valid?