Add command to print out all names in a MIR JSON file

[Ryan Scott]

Currently, there is no good way of determining all of the names defined in a MIR JSON file used in a SAW proof, short of manually inspecting the innards of the JSON. This is annoying, as it makes it difficult to know what names are in scope in a particular program. One can call print <mir-json-file>, but this will print out the full definitions of everything in the program, which is usually more context that what you want.

A better alternative would be something like mir_print_names : MIRModule -> TopLevel (), which prints out the names of each identifier and nothing more.