Mir: Trans of casts unimplemented

[nikomatsakis]

We need to support the various kinds of casts. This mostly involves looking at the old trans code for casts and porting it over to MIR, I imagine. The relevant unimplemented! is in trans/mir/rvalue.rs. Note that there are two: one for when trans'ing into a particular location (which would be used e.g. for casts to fat pointers) and one for when trans'ing into an Operand (e.g. for integer casts).

[KiChjang]

Is this issue up for grabs? If so I'd like to see what I can do about it.

[nagisa]

@KiChjang I already have code somewhere for Misc cast-kind somewhere around, which I’ll rebase onto master in a moment. Everything else seems to already be implemented.

[nagisa]

This is fully implemented to my knowledge (baring bugs) and thus can be closed.