Basic BWT en/de-coding implementation

Basic BWT:
encoder is Radix1 + built-in sort
decoder is either standard with extra N words, or brute-force N^2 complexity without extra memory (optional, to be optimized)

Tests pass, even though I had to comment out "#[deny(missing_doc)];" for them to compile ("rustc --cfg=test lib.rs").

We also need a basic app for stream encoding/decoding, using all the methods/options implemented in this lib.