Cryptic saw-verifier error when ghost state contains values of different types

[langston-barrett]

In the same vein as #542 when run on test_ghost_branch_03 from #541, saw-verifier yeilds this message when the ghost state has different types along different branches:

[22:17:09.062] Checking proof obligations h ...
== Anticipated failure message ==
user error ("crucible_llvm_verify" (/home/siddharthist/code/saw-script/intTests/test_ghost_branch_03/test.saw:36:10-36:30):
"z3" (/home/siddharthist/code/saw-script/intTests/test_ghost_branch_03/test.saw:36:71-36:73):
Verifier.SAW.Simulator.Prims.iteOp: malformed arguments: [_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_] <<boolean>>)
[22:17:09.065] done

Instead, we should disallow storing values of any type in ghost state, and make the declaration of a ghost variable explicitly typed.

[brianhuffman]

Fixed by #550.