Get rid of "get" prefix because it conflicts with getters

This is (hopefully) the last straggler in the 'get*' family, after getKey() and getAugmentation(). Using "get" prefix followed by a capital letter infringes on namespace reserved for user-defined data, such as:

grouping foo {
    leaf implemented-interface {
        type string;
    }
}

We suddenly have two getters differing only by return type, which is a JLS violation. Get rid of this by renaming to "implementedInterface".