`rank` and`select` on `WaveletTree`

[somethingelseentirely]

Most wavelet tree implementations extend the definition for rank, select, and access over bit-sequences to the wavelet tree itself.

• The WaveletTree::lookupoperation seems to be equivalent to a select with some additional caching done to accelerate subsequent queries.
• The WaveletTree::decode_one operation seems to be equivalent to an access operation.
• rank doesn't seem to have a corresponding method.

Since the implementation of this project appears to be an independent invention of the wavelet matrix it probably makes sense to just implement the rank from the paper. (Doing a comparison of the other operations is probably also interesting.)