Better naming and converntion for ExpressionContext

[pipermerriam]

#55 introduced ExpressionContext which isolates the mutable components of expression validation.

This name is still not sitting right with me. I don't like the name collision with the Context object. I think renaming it to ExpressionValidator would be acceptable, but then that leaves the question about what should be done with the various APIs that are on the Context object which need to be accessed during expression validation.