Feature: TakeSlice method

[Andrew-Morozko]

TakeSlice() allows to retrieve the underlying buffer as a slice, with all elements placed in the correct order and extra capacity preserved. It clears the internal state of the deque.

There are a few optimisations possible if you are ok with wasting some capacity at the start of the buffer (for example this one) and using 3 rotations is not the most optimal way to reorder elements, but it's simple and it works.

Perhaps an extra method that copies the deque into a new slice may also be useful, but this one seems semantically simpler