Rust syntax - Separate ControlFlow from Keywords

The syntax definition for Rust did not differentiate ControlFlow-specific Keywords from other Keywords, unlike those of other languages.

Maybe this is intended and, in that case, this MR should be rejected, but I personally find Rust code easier to read like this and this is more consistent with other languages, hope others would agree.

The "for" keyword is sometimes used outside ControlFlow, but this is not different from "default" in c++, which is highlighted as ControlFlow despite being used in member function definitions. It is still a good thing in my opinion.