Implemented bit strings

* BIT(1) gets mapped to bool, other bit types to .NET BitArray (Npgsql's
  BitString no longer used)
* To implement the above, had to allow FieldType to vary based on the
  field description's TypeModifier, and to refactor the array handler
  hierarchy to add a special one for bit strings.
* Note: did not reimplement complex bitstring text decoding, which assumed
  that hexadecimal and binary representations could be mixed in the same value.
* Moved type tests to their own folder