Add generics to function pointers

[kyleheadley]

The latest generics PR (#639) ignores function pointers and allows their void* params to spread wildness. We would like to mark these as generic when possible, but there are more complications than for regular function decls. We need to research the syntax. We need to constrain them against assignments of functions that may not be rewritten to generic. We need to allow alternate generic indexes as long as each group of indexes is consistent and equatable. This shouldn't be too hard for single-void rewrites, but more generic parameters will complicate it.

Likely low priority.