When an explicit parsing error is encountered and a CompileError raised,
apply the changes to the context state. This way, the rule that catches
the exception (e.g. Statement or Until) knows where the error occurred.
Also, changed "Expected" errors to be reported at the end of the
previous non-whitespace token.
Use a '$' instead of a '.' to indicate a type provided in application
code.
The reason for the change is to have a consistent "extern" symbol that
isn't widely used elsewhere and isn't ambiguous in expressions.
Inline menus didn't work anyway--menus have to be referenced by ID
(though, curiously, you *can* put the <menu> within the <property> tag
and immediately reference it--but that's a hack, and not what
blueprint-compiler was doing).
I want all language elements to be defined in the same folder, rather
than having the "core" language split by parsing/AST/completions and the rest
split by language element.
