Adapt PC extractor to work with library functions

[asymmetric]

This can't currently extract the PC value for internal library functions.

[asymmetric]

@livnev is this still relevant in light of #380?

[livnev]

@asymmetric I think it would be nice to try a test case for klab using libraries (we don't need to add it to the CI tests of course), so that we can prove that it works provided you are using dapp to build. There could still be other things that break with libraries.