See e.g. Applicative instance in Data.Vect. This allows implementations
to use implicits at run time (by default, they'd be 0 multiplicity so
erased, but it might be useful to have an index available at run time).
At the moment, the parser requires implicits to be given before
constraints. Ideally it should be possible to give them in any order.
I'll come back to this.
This is part of what we used to have in Enum but I think it's better to
separate the two. Added implementations for Nat, and anything in
Integral/Ord/Neg, so that we get range syntax (at least when its
implemeted) for the most useful cases.
