Allow storing non-static data in a vm

[Marwes]

Types with lifetimes may be possible. I actually had that working way, way back but ended up removing it since it was so limited (any lifetime needed to strictly outlive the interpreter itself making it more trouble than its worth).
Thanks to some changes done in the last year it could be worthwhile to try it again however as it is now possible to create short lived child interpreters which are already have the capability to restrict short lived values from escaping to longer lived interpreters. Piggybacking on this system it should be possible to create a interpreter for each script call (the interpreter could easily be recycled to avoid allocating a fresh one each time), pass it some reference values to the script and then destroy/recycle the interpreter before the references lifetimes ends. No promises though :)

From amethyst/amethyst#271 (comment)

cc @endeav0r