feat(lua): add vim.func._memoize

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'.

runtime/lua/vim/_meta.lua

@@ -10,6 +10,7 @@ vim._watch = require('vim._watch')
 vim.diagnostic = require('vim.diagnostic')
 vim.filetype = require('vim.filetype')
 vim.fs = require('vim.fs')
+vim.func = require('vim.func')
 vim.health = require('vim.health')
 vim.highlight = require('vim.highlight')
 vim.iter = require('vim.iter')