Implement conversion from non-det transitions to enumeration

[lopsided98]

Implement From conversions from non-deterministic transition enums to the state enum, if enabled. This makes it a lot nicer to work with non-deterministic transitions.

I've never worked on a proc macro before, so I'm not sure if my implementation is the best.

[jmg-duarte]

Hey @lopsided98, thank you for investing some time and contributing! Before I proceed with review, could you provide some tests over your feature? You can take a look at the existing ones in /tests.

Let me know if you have further questions!

[lopsided98]

I added a simple test. Let me know what you think.