Fix exp function to actually work

[djrodgerspryor]

It was calling into decNumberExp with the wrong signature, so it didn't work at all.

I also fixed the decNumber tests for exp so that they'll actually run if point at the exp.decTest file, and I added a super basic test for ln and exp.

It seems a bit too easy to not run some of the decNumber test by accident right now; maybe it would be good to expect every test file to have some non-igored tests, and then just delete the test files for actually unimplemented functionality?