Have BitField store ranges instead of bytes, and add benchmarks

[timvermeulen]

The main change: remove the RlePlus struct that held RLE+ encoded data, and instead have BitField store Vec<Range<usize>> directly. Specifically:

• when deserializing a bit field, instead of only verifying the data and keeping it as RLE+ encoded bytes, we verify the data by decoding it to Vec<Range<usize>> and store those ranges instead
• when iterating a bit field, the data no longer needs to be decoded on the fly — the ranges are iterated directly

As a result, deserializing a bit field is now about 1/3 slower, but deserializing + iterating a bit field is about 2x faster (and every subsequent iteration is much faster as well).

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Furthermore, improved ergonomics: added a Peekable struct just like std::iter::Peekable, and have the Union / Intersection / Difference iterators store those instead of storing the iterators and next ranges separately. Peekable distinguishes between not having looked at the next range and there not being a next range, so this allowed us to remove all FusedIterator bounds.

Other changes:

• add a couple of benchmarks that we can measure future changes with
• BitField::first now has a custom implementation that does not allocate
• BitField::get is made a lot faster by doing a binary search on the range vector

[ec2]

Optionception 🧠 💥