Memoizes a function, using a custom function to hash the arguments.

Private for now until:

- There are other places in the codebase that could benefit from this
  (e.g. LSP), but might require other changes to accommodate.
- Invalidation of the cache needs to be controllable. Using weak tables
  is an acceptable invalidation policy, but it shouldn't be the only
  one.
- I don't think the story around `hash_fn` is completely thought out. We
  may be able to have a good default hash_fn by hashing each argument,
  so basically a better 'concat'.

* feat(lua): allow vim.wo to be double indexed

Problem: `vim.wo` does not implement `setlocal`
Solution: Allow `vim.wo` to be double indexed

Co-authored-by: Christian Clason <c.clason@uni-graz.at>

Enforce consistent terminology (defined in
`gen_help_html.lua:spell_dict`) for common misspellings.

This does not spellcheck English in general (perhaps a future TODO,
though it may be noisy).

Problem:
  Codebase inconsistently binds vim.api onto a or api.

Solution:
  Use api everywhere. a as an identifier is too short to have at the
  module level.

Closes https://github.com/neovim/neovim/issues/18225