Add conversion to constants

[celinval]

Implement the translation MIR -> SMIR for constants.

[ouz-a]

@rustbot claim

Edit: Since bot doesn't work I'm going to work on this

[oli-obk]

The bot doesn't work yet

[ouz-a]

Ah, okay 😅

[celinval]

@ouz-a I assigned you for now.

[ouz-a]

I think we can close this issue now.