Add compatibility with bitvec's `BitSlice` trait

[hexane360]

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This adds an optional (disabled by default) feature, which when enabled adds an implementation of bitvec's BitSlice trait. It aims to add some cross-compatibility to the two crates, as well as allowing a variety of methods that operate on slices of bits.

The most controversial part here is adding a Deref implementation from FixedBitSet to &BitSlice. I think this is justified though, and there is little overlap between the methods provided by the two crates.