Add Enumerable(T)#to_a(& : T -> U) forall U

[caspiano]

Analogous to Enumerable(T)#(& : T -> Tuple(K, V)) forall K, V except producing an Array from the block's result.

Resolves #12643

[HertzDevil]

Indexable could override this to pre-allocate the returned array with capacity same as size. (This does not improve performance in all cases but is usually a right thing to do, like array literals.)

Also it is probably worth documenting that Array#to_a doesn't create a copy but Array#to_a(&) does.

[asterite]

Indexable could override this to pre-allocate the returned array with capacity same as size.

Strange, that's what Array already does. I'm surprised when Indexable was introduced this optimization wasn't also done.

This does not improve performance in all cases

Why not?

[HertzDevil]

If you immediately insert an element to the returned array then a reallocation is guaranteed to happen and that is less performant than the default reallocation strategy on average. This is why we didn't pursue #10075