Add more functionality to support succinct data structures

[bovee]

Looking at use cases for this library, it would be nice to support some more complicated succinct data structures. Our existing implementation is very similar to the int_vector<8> in SDSL so adding some of the rank/select/wavelet tree operations from that in here might be useful. See http://simongog.github.io/assets/data/sdsl-cheatsheet.pdf for details on the high-level functions in SDSL.

[boydgreenfield]

@bovee Agreed, that'd be very cool. Thoughts on just writing a wrapper for (a small portion of) SDSL? Note that it is GPL.

[bovee]

Having to include SDSL as a dependency for just a few data structures feels a little overkill (but it would be nice to have a general SDSL wrapper separately!).

My further thought here is to:

• add a RankSelect trait that can be "naively" implemented by BitVec (with rank and select functions)
• add an additional RRRIndex struct (e.g. http://alexbowe.com/rrr/) wrapping BitVec that implements optimized versions of those functions.
• add a WaveletTree that uses RRRIndex for its nodes (e.g. http://alexbowe.com/wavelet-trees/)

(This should maybe be an Epic)