This PR removes the `@[reducible]` annotation on `Array.size`. This is
probably best gone anyway in order to keep separation between the `List`
and `Array` APIs, but it also helps avoid uselessly instantiating
`Array` theorems when `grind` is working on `List` problems.
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.
