[CM] mp numbers in s7

[Bill Schottstaedt]

I've added support in s7 for multiprecision integer, ratio, real, and complex
numbers, including all the numeric functions.  Use the configure flag
--with-gmp.  It uses the gmp, mpfr, and mpc libraries.  Default float
precision is 128 bits, but can be set higher via bignum-precision.
Still a week or so of testing to do, but it currently passes the tests
in s7test.scm.  If this works out as I hope, I'll extend the mp arithmetic
to clm and snd.