Add comments and s/"/'/ for constant strings

Cleanup, style, and clarity, mostly.
Handle fixed-width values in PB::Binary::Reader

Handle 32- and 64-bit fixed width wire types, reading them as unsigned
integers.  This leaves just groups to implement amongst the wire types.
Tests for fixed-width wire types

Test reading 32-bit and 64-bit fixed width wire types, both "bare"
and as part of a kv field pair.