Implement unsized types

[bjorn3]

Working on it in the https://github.com/bjorn3/rustc_codegen_cranelift/tree/wip_unsized_types branch. Edit: merged in 42eeb75.

• unsized places
• unsized field projection
• unsize of adt's
• slices
  • unsize to array -> slice
  • Len(slice)
  • index slice
  • subslice
• trait objects
  • unsize trait object -> trait object
  • unsize to trait object
  • drop (done in 1eba7e5)
• size_of_val intrinsic
  • on slice
  • on trait object

[bjorn3]

Merged wip_unsized_types

[bjorn3]

🎉🎈🎉 Implemented trait objects 🎉🎈🎉