Disassembling constant literals is broken about rspirv

[supervacuus]

Hi, I had a brief look into this and wondered whether to fix it by adding signed/unsigned integer literals to the parser. Right now, the parser discards the sign of the tracked type when parsing literals:

https://github.com/gfx-rs/rspirv/blob/master/rspirv/binary/parser.rs#L312-L318

Generating signed integer literals at this point and correctly applying them throughout the code would fix this issue. The integer literals are already a specialization away from SPIRV's JSON grammar, so I don't think this change would break with spec. But introducing/changing literals might affect the parser's contracts to other code. If that direction sounds right, I can create a PR with my changes.

[supervacuus]

I've added a PR which only touches the disassembler. Modifying the parser as proposed above will break client code, if it expects matches on LiteralIntXX to mean all (= signed + unsigned) integer literals in the binary.

If breaking client code is okay for the convenience of no longer needing to track OpTypeInt, then I can create a very drafty alternative PR. But reading through your other issues, it might make more sense to handle this in SR rather than DR.