Add Block, Record, BlockInfoContext.

Also fix bug in _readFracBits when numBits is 32.
Use >>> for zero-fill shift
Add PEXE loading test.

Also fix bug in char6 abbrev op: it should return an array.

The pexe is loaded and compared to JSON generated by another program. As
a result, the data has to be munged. It was simplest to do that in the
test code, but it might be nicer to change the golden file.
Support reading u64 values via readVbr.

* Add U64 library based on Google Closure's Long. (It's almost a
  straight copy for now, but I'll probably modify it).
* Added readVbr32 which will only read 32-bit values
* Added tests for places where readVbr could potentially return a 64-bit
  value.