This PR modifies the order of arguments for higher-order `Array`
functions, preferring to put the `Array` last (besides positional
arguments with defaults). This is more consistent with the `List` API,
and is more flexible, as dot notation allows two different partially
applied versions.
This PR simplifies the signature of `Array.mapIdx`, to take a function
`f : Nat \to \a \to \b` rather than a function `f : Fin as.size \to \a
\to \b`.
Lean doesn't actually use the extra generality anywhere (so in fact this
change *simplifies* all the call sites of `Array.mapIdx`, since we no
longer need to throw away the proof).
This change would make the function signature equivalent to
`List.mapIdx`, hence making it easier to write verification lemmas.
We keep the original behaviour as `Array.mapFinIdx`.
