util.h: fix broken casts in Perl_instr() parameters

Why broken? Because haystack/needle are effectively macros (they expand
to token sequences), so a call like

    Perl_instr(a + b, x ? y : z)

expands to

    strstr((char *)a + b, (char *)x ? y : z)

which is just wrong.

Normally I'd simply add the missing parentheses ((char *)(haystack),
etc), but it's not clear to me why the casts were added in the first
place (and fairly recently, too, in commit 4e52881).

Background:

instr() was originally implemented in pre-standard C (without const and
without a prototype) in util.c, perl-1.0. The consts were added fairly
early on in 08105a9 (1997): char *
instr(register const char *big, register const char *little);

The hand-written C code was replaced by a call to strstr() in
5d1d68e. It cast away const from the arguments to strstr() for no
reason I can see: strstr() takes pointers to const char (just like
Perl_instr), so the pointers were immediately implicitly re-consted by
the prototype of strstr().

Commit fea1d2d straight up turned instr() into an alias for strstr(),
which was apparently fine without any casts for 4.5 years. The C code in
Perl_instr() was moved from util.c to mathoms.c in commit 534dad4.

Author: mauke

util.h

@@ -241,7 +241,7 @@ returning NULL if not found.  The terminating NUL bytes are not compared.
 */
 
 
-#define Perl_instr(haystack, needle) strstr((char *) haystack, (char *) needle)
+#define Perl_instr(haystack, needle) strstr(haystack, needle)
 
 #ifdef HAS_MEMMEM
 #   define ninstr(big, bigend, little, lend)                                \