sshbn.c: Turn the bignum arithmetic internals back-to-front.

sshbn.c: Turn the bignum arithmetic internals back-to-front.

The internal arithmetic used to be done on a big-endian representation
of the integers, while the external format was defined to be
little-endian.  This causes a bunch of indexing operations to be more
complicated than they really need to be, and, more importantly,
introduces an unfortunate mismatch between the two different strata of
the bignum library, which will be annoying later.  Finally, and least
important of all, in my very brief testing, the bignum test runs about
4.7% faster with this new version.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>