Implement Enums

[tillarnold]

Replaced by #24

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Builds on top of #16. To see what changed since #16 see https://github.com/tillarnold/prusti-dev/compare/rustfmt..tillarnold:prusti-dev:enum

This adds support for enums to new prusti, pure (snapshots) as well as impure (predicates).
I refactored mk_structlike to extract the different parts into functions that i can then reuse for mk_enum.

TODO:

• test pure (currently not possible due to goto issue)
• creating instances of enums in pure functions

[JonasAlaif]

This can't be used as is, please replay the changes without the cargo fmt once you have time :)

[tillarnold]

@JonasAlaif with all the other changes that happened to the type encoder I'm not able to just replay the changes on the main branch. I will recreate this manually on top of the current branch without the fmt run but it'll take a bit more effort than just a rebase.