D grammar overhaul

[Trass3r]

> As a starter, here are some syntactic problems in the ddoc sources that
> cause the generation of grammar.txt to fail:
>
> - in template.dd, $(GNAME Constraint) is not enclosed by $(GRAMMAR)
> - in attribute.dd, $(GRAMMAR $(B const)), $(GRAMMAR $(B override)), etc
> are used for formatting without having to do with the grammar. The boxes
> they generate don't make much sense anyway, so they can just be removed.
> - MulExpression is only in $(V1), so it does not show for D2
>
> I've attached the patch for these.

https://github.com/Trass3r/d-programming-language.org/commit/ae532130f0374079c93a2c8f0609f04705dd8f82