Move opcodes_with_metadata into a DiffOpcodeGenerator class.

This new class performs all the same functions as the original
functions. Some of the code has been moved out into helper functions
for organization issues, but the logic hasn't changed.

This is part of an effort to clean up the diff viewer code and make it
more extensible.

All unit tests pass.

Tested locally with some diffs I have that have moved content. Everything
worked as expected.