Idea for setting both mutables and immutables without allocation

[jeff.bezanson]

@ScottPJones

As far as BigInt (and BigFloat) are concerned, I think the only way to really get good performance is to move away from trying to use a type that points at C managed memory and to change things to use an immutable type, with Julia managed memory.

I completely agree, but we would get even better performance adding the ability to safely overwrite numbers instead of allocating new ones when possible, on top of that.