Don't rely on bignums in ntlm.el

Don't rely on bignums in ntlm.el

Since ntlm.el is distributed as a separate package in GNU ELPA and
should be able to run on older Emacs versions without bignums,
we cannot make use of them here.  See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2020-10/msg01665.html.
Instead, we add a small poor man's bignum implementation.

* lisp/net/ntlm.el (ntlm--bignat-of-int, ntlm--bignat-add)
(ntlm--bignat-shift-left, ntlm--bignat-mul-byte, ntlm--bignat-mul)
(ntlm--bignat-of-string, ntlm--bignat-of-digits)
(ntlm--bignat-to-int64): New.
(ntlm--time-to-timestamp): Use the ntlm--bignat- functions instead
of Lisp integers.
* test/lisp/net/ntlm-tests.el: New file.