As agreed in Prague (https://docs.google.com/document/d/1pgDY-RaTbajNqXm-oj8GdzPD_fPvLb2jhnrt...)
FieldDefinitionInterface is intended to be the "primary" interface for classes that implement it.
We should rename the methods from getFieldXxx() to just getXxx().
Also, badly needed after #2047229: Change notice: Make use of classes for entity field and data definitions , which forces "FieldDefinitionInterface extends DataDefinitionInterface", meaning classes implementing FieldDefinitionInterface now need to provide both getFieldXxx() *and* getXxx() :-(