Added automatic conversion and comparisons for base_uint<Bits, Tags>.

* Automatic conversions and comparisons are enabled both ways between
  base_uint<Bits, Tags> and base_uint<Bits, void>.

* No coversions or comparisons are enabled between base_uint<Bits, Tags1>
  and base_uint<Bits, Tags2> if Tags1 is not Tags2 and neither is void.

This is already successfully in use on another branch, https://github.com/rec/rippled/tree/x-ripple-calc.