Interval macro

This introduces the macro `@make_interval`

The user types arbitrary code and a correct bounding interval is created, e.g.

`@make_interval((2.1-pi)*(2.1+pi), pi*pi)`


I was thinking in using directly something like `convert(Interval,pi)` instead of `convert(Interval,big(pi))`; the second one works fine. Incidentally, I just noticed that `convert(Interval,0.1)` fails....