'forall' always a keyword, plus the dot type operator

This merge request:

1. implements the "Make forall a keyword in types" proposal (accepted)
2. implements the "The dot type operator" proposal (accepted)
3. fixes a bad error message about existential quantification, Trac #16311

It also improves handling of Unicode syntax in hintExplicitForall and adds two related utilities, starSym and forallSym.