Support exports without offset

[dureuill]

This PR is a replacement for #292, which applies the change discussed there:

• make offset an Option<_> in Export, and support when an Export has no offset.

Moreover, the correct commit of #292 (add missing subtraction) is retained.

Test

The same procedure as in #292 was followed. This change does fix the issue #291 and the previously missing symbols are now returned for the tested binaries.

Thanks again for your time and consideration!

[dureuill]

Ran cargo fmt and pushed again