Refactors resulting from the issue #83 review

Refactors resulting from the issue #83 review

Mostly quality of life improvements.

On the other hand, it looks like the notfatal hash table API was being
used incorrectly. HASH_ADD_KEYPTR can OOM, but `errno` wasn't being
catched.

Fixing this is nontrivial, however, because strange `reqs_error`
functions are in the way, and that's a spaggetti I decided to avoid.
Instead, I converted HASH_ADD_KEYPTR usage to the fatal hash table API.
That's the future according to #40, anyway.

I don't think this has anything to do with #83, though.