Add an error and adjust the terminology slightly about augmented redirecting constructors

[eernstg]

This PR introduces an error for the case where a redirection (this(...)) is augmented by another redirection, and similarly for a redirecting factory.

This follows the treatment of super-initializers, for which it is also an error to include one in an augmenting declaration if the augmented declaration already has an explicitly declared super-initializer.

Other than that, it clarifies the terminology of the augmentation feature specification slightly, such that we keep this(...) and this.x = 42 clearly separated.

'redirecting initializer' is changed to 'redirection' when talking about generative redirecting constructors. The term is derived from <redirection>, and the word redirection is used in other specification documents about this kind of term. This also helps avoiding the confusion about this(...) possibly being an initializer list element (like x = 42 or assert(b)).

I also added a few words of commentary right after the phrase 'factory redirection' because this phrase is new (the grammar rule actually doesn't have a separate name for the token sequence covered by this phrase). Finally, I deleted a TODO comment which is obsolete today (there are no special grammar rules about augmented).

[eernstg]

I assume this error was intended: "adds its redirection" sounds like the redirection position isn't currently occupied, otherwise we would say something like "adds its redirection or replaces the existing one". Also, the implied statement that the augmented constructor is potentially non-redirecting would not be true if it is already a redirecting generative constructor. Finally, it's an error to do the same thing with a super-initializer:

class A {
  A(int _);
}

class B extends A {
  B(): super(1);
  augment B(): super(2); // Error.
}

[eernstg]

Commented on two changes.

[eernstg]

Similar to the generative redirecting case.