parse field_expressions

[Tsche]

Currently it does not parse field_expression nodes. Those can be translated to getattr, ie:

metavar.foo; // -> getattr(metavar, "foo")
// also parsed:
metavar->foo;
metavar.*foo;

Whether -> and/or .* shall have special meaning in the DSL is to be determined. Right now I think it's best to raise a ParseError if these are used.