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'.

scripts/lua2dox.lua

@@ -361,6 +361,12 @@ local function process_line(line, in_stream, generics)
     return process_block_comment(line:sub(5), in_stream)
   end
 
+  -- Hax... I'm sorry
+  -- M.fun = vim.memoize(function(...)
+  --   ->
+  -- function M.fun(...)
+  line = line:gsub('^(.+) = .*_memoize%([^,]+, function%((.*)%)$', 'function %1(%2)')
+
   if line:find('^function') or line:find('^local%s+function') then
     return process_function_header(line)
   end