clean up api and tests a bit

- dropped 'codable statistics' ; complicated codec bin 2, wasn't
well implemented, wasn't implemented in any other 'codec'
- made codec an interface since its only non-abstract method was
overridden in all but one class
- dropped 'codable object reader/writer' ; unused, better ways to
do that if it is desired in the future
- tons of unit test clean up ; split up some of the larger mega
boolean to assert true checks, but tests could still use a bit of
work.
dropping FieldConfig.intern

could definitely be useful in some cases, but can't find any
current usages, and simplifies things for now
add little doc to FieldConfig