Simplify Enumerable#to_a

[straight-shoota]

The implementation of #to_a is identical to its yielding variant #to_a(&). We can simply delegate to #to_a(&).
As a bonus, inheriting types which override #to_a(&) with an optimized implementation will implicitly use the same optimization for #to_a as well. This means we can drop Indexable#to_a.

Inheriting types Tuple and Hash already explicitly delegate #to_a to their override implementations of #to_a(&).
We probably want to keep these overrides of #to_a because they augment the documentation.
But we can replace the method bodies with super to make it clear that the behaviour is inherited and the def only provides documentation.