[phobos] BigInt from Tango

[Walter Bright]

Don Clugston wrote:
> I've converted my BigInt module from Tango to Phobos2. It's a great
> test of the new operator overloading!
>   

Great!

> After 2.042 comes out, I'd like to add it to Phobos.
>   

Sure.

> A couple of issues:
> (1) We need somewhere in Phobos for implementation code (of which
> there is a considerable amount for BigInt).
>
> I propose std.internal.math.XXX; for all math related modules. This
> would keep everything tidy and hidden.
> None of this will be user-visible.
>   

How about a more general scheme? For internal implementation modules, 
just prefix an "internal" package to it, so we would have:

   internal.std.bigint
   internal.std.math

etc.