Add a “view” type for sequences of integers, to allow bitwise operations without copying

[BlacklightShining]

BitVec lets you manipulate individual bits, which is often useful, but it doesn't really help if you have a [u8] (especially a large one). Adding a sort of BitVecView type that could be created from a reference to a slice of integers and would behave like a BitVec using the original slice for storage would enable this.