Use MARC::Record#fields in each_matchin_line and optimize what used to be spec_covering_field
Comment out scf and prepare for rough benchmarking
Put logic to find matching spec into #get_matching_spec
Reintroduce link to jruby bug that requires weird encoding/byteslice
Rework spec_covering_field, add more comments
Jeez, Set is slow. Use a hash
Switch to using hash for @interesting_tags
Merge in hash code
Merge branch 'scf' of github.com:billdueber/traject into billdueber-scf
MarcExtractor, remove extra spec[:indicators] check, that's built into #matches_indicators already, much simpler code.

If I have somehow eliminated perf improvements by adding in the
extra method call, I guess we can reconsider? But the duplicate
code checking the same thing in both places rubbed me the wrong
way, and this way Ive committed makes the code read much better.
MarcExtractor, no need to do a hash check to short-circuit avoid mostly just a further hash check later! The one hash check does it.