Rename Parallel => ParBeta or FullBeta.Parallel #990

#965 introduces parallel η-reduction. As a result, Parallel is no longer an unambiguous name for the parallel β-reduction, since we will now have both parallel β-reduction and parallel η-reduction.

The parallel β-reduction is currently defined as Parallel.

I suggest renaming Parallel to either ParBeta or FullBeta.Parallel. This would make it clear that the relation specifically represents parallel β-reduction.

There is also a notation issue. Parallel currently uses the notation ⭢ₚ. If ParEta is introduced for parallel η-reduction, it is unclear what notation should be assigned to it.