Add container::{NewContainer, Seq, MutableSeq} and other cleanup

I expect this will need some discussion before it lands. This PR starts fleshing out `std::container` with three new traits. The first, `NewContainer` allows someone to generically create of a certain size. The second is `Seq`, which is a placeholder for generic immutable sequence methods, but it's currently empty. The third, `MutableSeq`, is where mutable functions like `.push()`/`.pop()` and etc can be placed. For the moment only `vec` implements MutableSeq.

It also includes some minor cleanup, like standardizing some comment doc styles, and adding a `.clone_iter()` to vecs so we can change `vec.push_all` into taking an iterator.