Bignum support added to XLISP 2.1h

[grimm at MIT.EDU]

Also in the area of "bignum support", I have developed a C++ class for
large integers.  I haven't made in solid speed measurements yet, but
it seems to be relatively fast.  (More precisely, I was able to
compute large Fibonacci numbers much faster than in scheme running
under emacs.  I don't claim this to be thorough measurement in any
way!)  If anyone is interested, send me email.

-James