Add 'from common_func import ...' support

[garyttierney]

This is still a WIP so I've marked it as a draft. There's no error handling yet and the discovery of common_func is a little out of place, but it does the trick for me so far in DS3.

Still todo:

• Importing events listed by wildcard or name in from common_func import *
• Importing events qualified by module in import common_func
• More robust error handling
• Add some docstrings
• Decompiler support

Do you think this is the right time/place to be reading custom_func.py and parsing it's AST (i.e. on demand as opposed to passing in a load_common_func=True to an EMEVD instance).

[Grimrukh]

This is looking good. Yes, I think this could be easily expanded to import event functions from any file - we'd just need to tell the compiler that only the common_func ones don't need to be inlined in the final EMEVD.