Support unsized types

[RalfJung]

One of the things I am least sure about is how to best add support for unsized types to MiniRust. Clearly Pointer should optionally carry some metadata, but what do we do about all the places in the spec where we make use of the size (or alignment) of a type? There will be quite a few decisions to be made there, I think. Unsized arguments will also be interesting; unsized locals in general I think are out-of-scope for now due to their various problems.