MIR: Support raw pointer types

[Ryan Scott]

The SAW MIR backend currently supports writing specifications involving reference types, but it does support specifications involving raw pointer types (e.g., *const T or *mut T). This would be of benefit for certain classes of unsafe code. In particular, it is important to support the read_volatile and write_volatile functions, which are used for ensuring memory sanitization. (See also GaloisInc/crucible#1130, which would be a prerequisite.)

Note that crucible-mir itself already supports raw pointer types, so fixing this issue would likely be a simple matter of determining what API we want to offer in SAW to interface with raw pointers.