bitarray indexing: sync with array + refactor

* Use setindex_shape_check and DimensionMismatch instead
  of generic errors; this (and a couple of extra minor fixes)
  makes BitArrays behave like Arrays (again)
* Move portions of the indexing code to multidimensional.jl
* Restrict signatures so as to avoid to_index and convert, then
  create very generic methods which do the conversion and the
  dispatch: this greatly simplifies the logic and removes most
  of the need for disambiguation. Should also improve code
  generation.
bitarray: more systematic indexing tests
bitarray: small code simplifications

follows change in bitshift operators behavior
(i.e. saturating instead of modular)