RFC: Basic BigRange support available

[andrioni]

Currently there is a method for colon that makes all BigInt ranges BigRanges. It does incur in some overhead because of the BigInt used for managing the size. If this is fine, I'll extend it to also BigFloat ranges.
Fixes the second part of #3079.

[andrioni]

@JeffBezanson @StefanKarpinski ?

[staticfloat]

Is this still applicable? Should we try to get this in before 0.2?

[ViralBShah]

I would prefer to have this post 0.2.

[StefanKarpinski]

I agree. I think there's something better to be done for ranges in general, but it's a bigger change.

[andrioni]

This would help with #4845, since it'll allow ranges like big(0):typemax(Int) to be valid. Any ideas on a better solution?

[JeffBezanson]

My branch jb/newrange1 has a more general solution to this.