Anonymous delegates

Final implementation for issue #658.

This PR implements anonymous delegates to be exclusively used as parameter types. They are a simplified expression for named delegates, but do not support all their language features. Inner parameters are anonymous (without identifier) to be even more short-hand.

void f (delegate(int, int) => int d) {
   stdout.printf ("%d\n", d(1, 2));
}

void main () {
   f ((a, b) => a + b);
}

Restrictions are:

• no throws-clause, named delegates should be preferred in this case
• anonymous delegates cannot return an anonymous delegate, named delegates should be preferred in this case
• outer dynamic and params modifiers are not allowed, as they would require arrays of anonymous delegates and run-time information that is unavailable to this implementation
• outer ref and out modifiers are not permitted as they would require the anonymous type to be reused elsewhere
• inner ref, out, unowned and weak modifiers are allowed
• no inner dynamic and params modifiers, named delegates should be preferred in this case
• inner optional parameters are not permitted as the argument would need to be named and not anonymous to retrieve the default value
• inner and outer code attributes are accepted, but a warning is thrown if they are ambiguous (void f([CCode (scope = "async")] delegate() => void d) could apply to the parameter d or the anonymous delegate; in doubt it is always applied to the parameter)

Moreover, the following functionalities come along:

• warning saying that anonymous delegates are still experimental
• meaningful error messages
• pretty printing of anonymous delegates in compiler messages
• updated (and tested) CodeWriter
• 20 tests