Consider using a mapping lookup in F_SWITCH when case ranges are not in use.

array.c:switch_lookup() currently uses binary search to perform the lookup. In the common case where no F_CASE_RANGEs are used it would be possible to just use a mapping lookup instead.

This would require a slight API change for switch_lookup() in that the first argument would need to be a struct svalue * (instead of the current struct array *).