fix: Optimize use of hash tables in platform adapters

[mwcampbell]

I planned to do this after I changed accesskit_consumer to use hashbrown. I haven't bothered to do this for accesskit_atspi_common or accesskit_unix, since interoperability with zbus is a concern for these crates, and the cumulative binary size of the Unix adapter is already quite big.

Also, for the macOS adapter, I decided that using a hash table to keep track of view subclasses was overkill, since the number of items will always be small, and lookup will be rare. So I decided to favor simplicity and minimal code size instead, by using a Vec with linear search. The other reason to revisit this was to eliminate the dependency on once_cell.