[bitArray] some basic unit testing
[bitArray] add byteswapM function
[bitArray] reduce amount of copying, add clampM and shiftLeftM

* new functions: clampM, shiftLeftM, _clampLastM, _shiftLeftM
  modifying the passed array instead of copying it over and over
* dropped _shiftRight
* reworked bitSlice, concat, clamp to make use of the new functions,
  minimizing amount of copied ("sliced") data
* force mask in partial - drops bits that are not included in length
[bitArray] define internally used functions in closure

* calling local functions is faster
* better compression (local names get shortened)
* docs still working
* also fixed some docs