DIP9 -- Redo toString API

[bearophile]

Don:

> The efficiency issues are important, but are not the primary motivation.
> toString() is just wrong. The idea that there is ONE AND ONLY ONE 
> textual representation of an object, is, frankly, idiotic.

To solve that the toString() may take an argument, like a formatting string or an enum, given to it by the to!() and writef(). (But I understand the idea with the sink is more efficient).

Bye,
bearophile