Add a GDB Python script to pretty-print Bignum.

I've been playing around with GDB's Python scripting system recently,
and this is a thing I've always thought it would be nice to be able to
do: if you load this script (which, on Ubuntu 18.04's gdb, is as
simple as 'source contrib/gdb.py' at the gdb prompt, or similar), then
variables of type Bignum will be printed as (e.g.) 'Bignum(0x12345)',
or 'Bignum(NULL)' if they're null pointers, or a fallback
representation if they're non-null pointers but gdb can't read
anything sensible from them.