WyC: Partial fix for #427.

This fix introduces the mustParseAsExpr() function which determines
whether or not a given expression must be an expression, or could
possibly be a type.  This is needed to disambiguate cast expressions in
some cases, as identified in #427.  Currently, the fix is not complete
as a small number of tests are still failing.
WyC: Completed fix for #427.

This completes the fix for #427, allowing a wider range of cast and
bracketed expressions to be disambiguated.
Merge branch 'feature/427-casts'