Refactor Array::slice_as_chunks(_mut)

[tarcieri]

Moves them from being free functions to inherent methods on Array.

This avoids having to import them separately.

[tarcieri]

@newpavlov if you'd like to leave these as free functions I'm fine with that too. There's certainly precedent for that sort of thing in core::array.

[newpavlov]

I am fine with both options, but the inherent methods look slightly more ergonomic. Not only you do not have to explicitly import them, but they also can simplify type hinting, i.e. we will be able to write Block::slice_as_chunks(buf).

[tarcieri]

Yeah, that should be useful