add new vector representation as a library

This is just an initial implementation and does not yet fully replace `~[T]`. A generic initialization syntax for containers is missing, and the slice functionality needs to be reworked to make auto-slicing unnecessary.

Traits for supporting indexing properly are also required. This also needs to be fixed to make ring buffers as easy to use as vectors.

The tests and documentation for `~[T]` can be ported over to this type when it is removed. I don't really expect DST to happen for vectors as having both `~[T]` and `Vec<T>` is overcomplicated and changing the slice representation to 3 words is not at all appealing. Unlike with traits, it's possible (and easy) to implement `RcSlice<T>` and `GcSlice<T>` without compiler help.