Implement lexcompare

[ppenzin]

Fort emits calls to lexcompare, but that always returns 0:

template<typename T>
static int32_t lexcompare(const CharacterValue<T> LHS,
                          const CharacterValue<T> RHS) {
  return 0;//FIXME
}

lexcompare is produced when two strings are compared usind .lt., .gt., and so on. In Fortran that requires padding shorter string with spaces before comparison.