Currently, strchrnul() is implemented as strchr() and falls back to a
strlen() to find the null-terminator if the character is not
found, scanning the string twice. However, strchr() is going to scan the
whole string anyway and discard the pointer to the null-terminator if the
character is not found, returning NULL.

Instead, we can just implement strchr() with discarding strchrnul()'s
pointer to null-terminator returning NULL and by that avoid calling
strlen() in strchrnul() if a character is not found.

I made a typo in the strchr(), it should be:
  return s1 && *s1 ? (char *) s1 : NULL;
which should be:
  return *s1 ? (char *) s1 : NULL;
since strchrnul will never return NULL.

We can avoid the strchrnul() function call in strchr() by implementing it
in a separate header like str-two-way.h and including them in both files.
The same could be done with the strlen() part in strchr() since the
implementations look to be the same.