Add a BitVecView<'a> that references the BitVec with offset / len

[kamalmarhubi]

This is related to #13, but I think it can be implemented before IndexGet by adding

impl BitVec {
    fn slice<'a, R: RangeArgument>(&self, offset: usize, len: usize) -> BitVecView<'a> { ... }
}

Thoughts?

[Gankra]

RangeArgument is still unstable afaik, so it would have to be behind the nightly flag.

Otherwise it seems fine; I'd call it BitSlice.

[pczarn]

The mutable view &mut BitSlice could help with issue #43

The problem is, we do not have stable CoerceUnsized, Unsize or any solid DSTs story in Rust as of today so that we cannot yet implement this cleanly.

[pczarn]

I would prefer to implement this as &'a BitSlice and &'a mut BitSlice

[pczarn]

Similar to #29