add a hybrid unbounded integer type to the stdlib

[thestinger]

It would use the checked overflow traits to expand the representation to a big integer on overflow. It will be much slower than a bounded integer type, but not as slow as a big integer before overflow (but slower afterwards).

This is similar to an implementation of small vectors.

[alexcrichton]

This might belong better in libextra instead of libstd. Plus there's already a bigint in there which could be used for the "extra large" integers for this.

[thestinger]

Blocked by #11536.

[thestinger]

I don't think we should include big integers in the standard library if it's not a competitive implementation. As the only realistic option is GMP and it isn't viewed as acceptable, I no longer want to do this.