diff for RELs

[robojumper]

It would be nice to have something like ninja diff working for RELs. A use case can be found in https://github.com/robojumper/ss-dtk/tree/rel-wip (SOUE01) - with the latest commit on that branch, objdiff shows no difference, but checksum verification for the d_t_tackle REL fails for not immediately obvious reasons.

The reason in this case is that two virtual function overrides were (accidentally) swapped in the symbols, which resulted in an incorrect vtable after linking, but to figure this out I had to binary diff the whole REL in a hex editor - which is doable for a small REL like this but scales poorly.