feat(lua_iteration): Add explicit support for iterating over parts of the API

[AndreLouisIssa]

Add explicit support for iterating over parts of the API

Namely (in order of most useful to least):

• Make gm's gm.call __index contributions iterable (could be gm.functions)
• An iterable wrapper for global variables (could be gm.globals)
• An iterable wrapper for a CInstance's instance variables (could be CInstance.variables, this can be done purely in lua but would not need marshalling if it had built-in lua-value translation for RValues)
• __pairs and next support in general for sol objects (this can be done purely in lua, but it is awkward)
• __pairs and next support in general for enumeration tables (it can be done purely in lua since their __index is a raw table)

For most of these, instead of separate iterable objects they could be done by extending __pairs and next on the existing structures.