Implement the rest of the possible casts

[solson]

Cast kinds:

• Unsize from array pointer to slice pointer
• Unsize to trait object pointer¹
• Reify function pointer
• Everything else²

¹ This goes along with implementing vtables and virtual method calls.

² Currently casts with same-sized source and destination are implemented by a blind copy and casts from fat pointers copy just the pointer part. This works well enough to pass most of our existing tests but it's extremely hacky. Search for the Misc case.

This will likely require multiple PRs.