Use a `Storage` backend for collection rather than an allocator-based one

[TimDiekmann]

At internals.rlo a nice proposal was made. I like to track the proposal in this repository but keep the discussion at the linked topic to avoid splitting the context.

[programmerjake]

I mentioned this in the latest ArrayVec RFC:
rust-lang/rfcs#3316 (comment)

Also, the conversation for storages has apparently moved to:
#93