New contrib: SB-GMP

New contrib: SB-GMP

 This contrib was developed by Stephan Frank to replace some of our
 bignum and rational arithmetic code with calls to libgmp.  Simply
 loading the contrib will transparently accelerate arithmetic on
 large rationals when libgmp is available; if libgmp cannot be found,
 the contrib should change nothing.

 The contrib also wraps additional functions in GNU MP, so that they
 accept and return SBCL-native integers or ratios.  See GNU MP's manual
 for more information.