Use newtype enums for representing frontend and backend jobs

[Kobzol]

I found the Option<NonZero<usize>> representation quite confusing. This PR replaces it with strongly typed enums, which helps with creating specialized methods (is_synchronized), and documenting what do the individual variants mean, rather than just working with Some/None and calling is_some().

Otherwise, the behavior should be unchanged.

I think that the MaybeParallel variant could also be named Synchronized, but there is no perfect solution here to really express what it means.

r? petrochenkov

[petrochenkov]

So much boilerplate and we still convert the "newtypes" back to an option in most places.
This is exactly what I wanted to avoid originally.

View changes since the review

[Kobzol]

The actual representation is an Option<NonZero<usize>>, but I think that there is benefit to wrapping it, so that we make it easier to understand how exactly it behaves.

What about a compromise:

struct FrontendJobs(Option<NonZero<usize>>);

This allows us to document the behavior + add methods to the type to reduce confusion of how it behaves, vs just using is_none().