The first step in the ExpPatFrame refactoring:
* Create a new intermediate type, ExpPatFrame.
* Change Parser.y to produce ExpPatFrame for expressions.
* Write 'checkExpr' of type LExpPatFrame -> LHsExpr GhcPs.
* Modify 'checkPattern' to have type LExpPatFrame -> P (LHsPat GhcPs).
There are two independent directions for future work:
1. Change 'checkExpr' to work in the 'P' monad and perform validation.
2. Change 'checkCommand' to take ExpPatFrame as input.