DiffOpcodeBuffer wraps a stream of opcodes, yielding them one at a

time, but with buffering capabilities. These enable callers to peek at

the next opcode without consuming it or to queue up an opcode to return

before the next from the buffer.
This will be used in the upcoming work for the interdiff filtering

algorithm.
It also backports some of the diff opcode typing from Review Board 9.

Unit tests pass.