Add Hash derive to Type

[TommyAmberson]

Pretty self explanatory PR. I wanted to add codegen::Type to a HashSet but to do that cleanly it would need to impl Hash. I figured someone else might want to do something similar as well so I put together this PR.

[SebastienGllmt]

You may have better luck submitting your PR to https://gitlab.com/IovoslavIovchev/codegen since that's the one published on crates.io