Move the general expression parser over to a general interface.

To allow for any given expression parser to be used in jTET, have both
existing expression parsers implement an interface, instead of being a bunch
of static methods.  This also moves all code over to the new interface.

This simplifies ParserOperatorParseTest, as it no longer needs reflection
to call the parse functions.

This adds some extra special exceptions to report errors from the parsers.
The existing ones don't make use of them yet, but they can in the future
when error reporting gets better.