rm const_get usage: MutatorForClassificationFactory

I've decided to remove all lookup magic in line
with taking out all the `method_missing`.  While
the use of `const_get` wasn't too hard to reason
about, I want to try to be explicit wherever
possible.

This required moving some `require` statements
about and was helped along by adding in some short
lookup methods e.g. `short_name_key`.
cleanup todos list