Arithmetic type emulator helper class for testcases.

[Paolo Carlini]

Argh! ;)

I meant only that: 1- <date_time> must go away, as soon as possible; 2- If something else in the library currently relies on it (I don't think so, actually! Probably a bit misunderstanding came from this...) the first bits of <chrono> going in will be necessarily those providing such functionalities, because we don't want to break anything while we remove <date_time>.

Assuming both 1- and 2- above are fulfilled, you are of course welcome to work on whatever you want, in any order, by any means.

Is the above more clear?

Paolo.