WIP: Lua mutation operators!

+=, -=, *=, etc ...

Thanks go to Ian Millington and Robert Jakabosky for writing the original patch!
Original code: http://lua-users.org/lists/lua-l//2011-06/msg01223.html

Legal notes:
Ian released the code under the MIT license (same as Lua 5.0), which is GPL-compatible..
I'm pretty confident it's legal to their code in SRB2.

Have a test Lua: mutationoperatortest.lua

WIP because it can print unhelpful errors when there's multiple lvalues