Improve speed of string comparisons.

Before this change:
  Running test string-compare-with-defaults-benchmark: PASSED in 13.395591s
  Running test string-compare-ic-benchmark: PASSED in 16.162011s
After this change:
  Running test string-compare-with-defaults-benchmark: PASSED in 1.093162s
  Running test string-compare-ic-benchmark: PASSED in 1.270290s

About half of the improvement came from adding a method on
%string-compare for byte strings, and half came from inlining the
standard comparison functions, char-compare{-ic}.