ucode-mod-lua: support prototype lookups and method calls on ucode values

Expose ucode arrays and objects with prototypes as userdata proxy objects
to Lua and extend the userdata metadatable with an __index metamethod to
lookup not found properties in the ucode values prototype chain.

Also extend the __call metamethod implementation to infer method call
status from the activation record in order to invoke ucode functions with
the correct `this` context when called as method from Lua.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>