std.bigint: BigInt conversion

[bearophile]

novice2:

> I hope, this changes will be included in phobos in future.

Sometimes hoping isn't enough. If you have a need that youi think 
should be in Phobos, then I think you should ask for it in 
Bugzilla. Asking isn't equal to someone actually implementing it, 
but in this early stage of the life of Phobos knowing what's 
needed is a very interesting information.

Bye,
bearophile