This adds a `mir_fresh_expanded_value` command, which creates a MIR value
populated entirely by fresh symbolic variables. For compound types such as
structs or arrays, this will explicitly set each field or element to contain a
fresh symbolic variable. This is heavily inspired by the
`llvm_fresh_expanded_val` command in the LLVM backend.

This command will be important for the MIR user story going forward, as we will
impose an requirement that all mutable allocations in an override must be
explicitly set in the override's postconditions. The `mir_fresh_expanded_value`
command provides a convenient way to quickly generate values to set in the
postconditions.

At present, `mir_fresh_expanded_value` comes with the limitation that it does
not support reference types. This is not an inherent limitation, but it is one
that will require some additional work to lift. In particular, we will likely
need something like a `mir_fresh_ref` command (similar to LLVM's
`llvm_fresh_pointer` command) to make this work. We leave the task of
implementing `mir_fresh_ref` to a later patch.

This checks off one box in #1859.