Optimize `1+' and `1-' on fixnums.

Optimize `1+' and `1-' on fixnums.

* libguile/vm-i-scheme.c (INUM_MAX, INUM_MIN): New macros.
  (add1, sub1): Add/subtract without untagging the operand.  This leads
  to a 44% run time improvement compared to the previous
  implementation.

* libguile/vm.c: Include <stdint.h>.

* test-suite/tests/numbers.test ("1+", "1-"): Add tests for
  MOST-POSITIVE-FIXNUM, resp. MOST-NEGATIVE-FIXNUM, for 32-bit and
  34-bit values thereof.

* benchmark-suite/benchmarks/arithmetic.bm: New file.

* benchmark-suite/Makefile.am (SCM_BENCHMARKS): Add it.