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)`


Actually `convert(Interval, 0.1)` doesn't fail; what fails is `show`, since the members are not `BigFloat`s. 
I'll modify `show` so that it checks to make sure they are `BigFloat`s to use `prec`.

In any case, I don't think there is any longer any necessity to use `convert(Interval, pi)` instead of `@interval(pi)`?