Added FIXNUM-SUB-AST for implementing FIXNUM subtraction.
Added FIXNUM-SUB-AST for implementing FIXNUM subtraction.

vtomole/SICL,clasp-developers/SICL,clasp-developers/SICL,vtomole/SICL,vtomole/SICL,vtomole/SICL,clasp-developers/SICL,clasp-developers/SICL