Fix/avoid re-entrance

[LlamaLad7]

Based on my other PR since I can't build otherwise. Mixin strictly disallows re-entrance during transformation so we should avoid evaluating args suppliers at transformation-time, instead storing the supplier and evaluating it at runtime. Currently if an arg supplier causes a classload for a class that has a mixin it will crash.

[MrNavaStar]

Running into this issue, merge would be great! (Thanks for the awesome library)