Eliminate use of StatementContextBase from parser.spi.meta classes

Prerequisite.get() is used across several yang-parser-impl classes which utilize InferenceAction. In some places, the returned StmtContext needs to be downcasted to StatementContextBase in order to perform necessary operations with it later.
It would be nice if the Prerequisite.get() method returned StatementContextBase right away so that we would not need to do the downcasting.