add ffi tests & fix ffi

[Skgland]

Ovehauls the ffi implementation.

• adds various tests
  • Fix invalid casts in is/2 #2777 (comment)
  • Fix invalid casts in is/2 #2777 (comment)
  • and many more
• replace a lot of low level libffi usage with higher level api usage
• adds type aliases so that rust type names for integer types can be used (float types already used f32/f64)
• add support for cstr in return types and when unpacking structs
• properly throw errors instead of crashing in several invalid egde cases
• properly handle some edge cases (e.g. integers that can't fit in a fixnum)

[Skgland]

I finally got this rebased and unless I forgot something I still wanted to changes this is now ready for review.

[Skgland]

With CI green after the rebase this is again ready for review @mthom