Use symbol name when comparing against an externed reloc

[r-burns]

For partial matching files, often a symbol is externed even though it should exist in the target object. We can still compare the symbol name, instead of always returning a mismatch.

I think this also fixes #210

[encounter]

Couple things:

1. Let's combine this with the (None, None) case
2. Could you also update the reloc_eq in diff/data.rs

[r-burns]

cool I think I got it, is this what you meant?

[encounter]

Yes, looks good, thank you!