implement a packed binary rational arithmetic type

[Ravenwater]

We have an (elastic) decimal rational type, but we do not yet have a binary rational type.

The design has some options but we would like to continue to follow the pattern where a programmer can define the size in bits and a storage type. A rational<nbits, BlockType> offers number systems that discretize the ratios, and with growing nbits provides finer and finer approximations to the real line.