crucible-mir: Improve pretty-printing of ZST constants

[RyanGlScott]

ConstZST (i.e., constant values with zero-sized types) do not include any information about what the zero-sized type is. In particular, function constants are represented as ConstZSTs, and the only way to determine what function the ConstZST corresponds to is to look at its type (FnDef), which stores the DefId of the function name itself.

Previously, the crucible-mir pretty-printer was printing ConstZSTs as <ZST> because it did not have access to any type information when pretty-printing ConstVals. This results in very suboptimal pretty-printed output (see the example below). This is also true for other forms of ConstZSTs, although currently mir-json only turns function constants into ConstZSTs. (In the future, this could conceivably change.)

To fix this issue, we add special cases in the Pretty Constant instance (which does have access to type information for ConstVals) such that ConstZSTs are printed using their type information. In particular, if the ConstZST has type TyFnDef, then the function DefId stored in the TyFnDef is printed. All other forms of ConstZSTs are printed like <ZST: [ty]>, where [ty] is the pretty-printed type.

As an example of the improvements in this patch, consider this program:

pub fn foo() {}
pub fn bar() { foo() }

Before the changes in this patch, the MIR for bar() would be pretty-printed as:

fn test/f3b25c00::bar[0]() -> () {
   let mut _0 : ();
   bb0: {
      call(_0 = <ZST>(), bb1)
   }
   bb1: {
      return;
   }
}

(Note the call to <ZST>(), which gives no indication whatsoever about what function was called.)

After the changes in this patch, it is pretty-printed as:

fn test/4717ccf0::bar[0]() -> () {
   let mut _0 : ();
   bb0: {
      call(_0 = test/4717ccf0::foo[0](), bb1)
   }
   bb1: {
      return;
   }
}

Fixes GaloisInc/mir-json#79.