Expose underlying data for BigUint and BigInt.

It would be useful to have access to the underlying data used to represent these numbers for use in certain algorithms that can be optimized with access to the underlying data as seen in:
ftp://ftp.cs.pdx.edu/smn/ipsec/skip/bnlib/jacobi.c

Perhaps another way to go about this would be to implement the Index trait for these two data types.
I'll put that in. I am wondering if this would be bad overall exposing the underlying data though, the more I think about it. For instance, what if we wanted to change from Big Endian to Little Endian, or in some other way change the underlying data format. Right now it is all nicely encapsulated, perhaps it is better to keep it that way and have people implement their own big integers if they need more functionality? I am not sure, but I'll make the change to expose slices instead of vecs.