This PR reviews the implicitness of arguments across List/Array/Vector,
generally trying to make arguments implicit where possible, although
sometimes correcting propositional arguments which were incorrectly
implicit to explicit.
This PR uniformizes the naming of `enum`/`enumFrom` (on `List`) and
`zipWithIndex` (on `Array` on `Vector`), replacing all with `zipIdx`. At
the same time, we generalize to add an optional `Nat` parameter for the
initial value of the index (which previously existed, only for `List`,
as the separate function `enumFrom`).
