Move the common code into a helper package akin to JLLWrappers.jl

[fingolfin]

This allows us to improve the wrapper packages by updating this shared package, instead of having to regenerate all wrapper packages each time we improve the wrapper code generator.

It might also improve precompilation and/or load times a bit (I think that was one of the motivations for JLLWrappers.jl), but that's at beast secondary for me right now.