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/func.lua

@@ -0,0 +1,41 @@
+local M = {}
+
+-- TODO(lewis6991): 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` is completely thought out. We
+--   may be able to have a good default hash by hashing each argument,
+--   so basically a better 'concat'.
+-- - Need to support multi level caches. Can be done by allow `hash` to
+--   return multiple values.
+--
+--- Memoizes a function {fn} using {hash} to hash the arguments.
+---
+--- Internally uses a |lua-weaktable| to cache the results of {fn} meaning the
+--- cache will be invalidated whenever Lua does garbage collection.
+---
+--- The memoized function returns shared references so be wary about
+--- mutating return values.
+---
+--- @generic F: function
+--- @param hash integer|string|function Hash function to create a hash to use as a key to
+---     store results. Possible values:
+---     - When integer, refers to the index of an argument of {fn} to hash.
+---     This argument can have any type.
+---     - When function, is evaluated using the same arguments passed to {fn}.
+---     - When `concat`, the hash is determined by string concatenating all the
+---     arguments passed to {fn}.
+---     - When `concat-n`, the hash is determined by string concatenating the
+---     first n arguments passed to {fn}.
+---
+--- @param fn F Function to memoize.
+--- @return F # Memoized version of {fn}
+--- @nodoc
+function M._memoize(hash, fn)
+  return require('vim.func._memoize')(hash, fn)
+end
+
+return M