HeArray: remove hek_hash and refcounted_he_hash

Calculate hashes on demand, but not store it in a HEK
to make HEK shorter to fill more entries into a cache line.
HEK_HASH(hek) is now invalid and gone.
Use the new HeHASH_calc(he), HEK_HASH_calc(hek), SvSHARED_HASH_calc(sv)
instead.
See http://www.ilikebigbits.com/blog/2016/8/28/designing-a-fast-hash-table
for benchmarks (HashCache).

And using 4 tests in the hot hash loop also makes not much sense,
when checking the length and the string is enough to weed out
collisions.
This strategy, recomputing the hash wehen needed, is so far 1-7% slower,
but we hope to get to speed with the HeARRAY patch. See below.

The endgoal is to get rid of linked lists and store the collisions
inlined in consecutive memory, in a HekARRAY. (len,cmp-flags,char*,other-flags,val)
Measurements in "Cache-Conscious Collision Resolution in String Hash Tables"
by Nikolas Askitis and Justin Zobel, Melbourne 2005 show that this is the
fastest strategy for Open Hashing (chained) tables.
See GH #24 and GH #102

The next idea is to use MSB varint encoding of the str length in a HEK,
because our strings are usually short, len < 63, fits into one byte.
We can then merge it with the cmp-flags, the flags only needed for comparison.
See https://techoverflow.net/blog/2013/01/25/efficiently-encoding-variable-length-integers-in-cc/
or just <63 one byte, >63 MSB: I32 len.
Note that the 1st MSB bit is already taken for UTF8.

Author: Reini Urban

dump.c

@@ -2709,7 +2709,7 @@ Perl_do_sv_dump(pTHX_ I32 level, PerlIO *file, SV *sv, I32 nest, I32 maxnest,
                         }
                         count++;
 
-			hash = HeHASH(he);
+			/*hash = HeHASH(he);*/
 			keysv = hv_iterkeysv(he);
 			keypv = SvPV_const(keysv, len);
 			elt = HeVAL(he);
@@ -2722,7 +2722,7 @@ Perl_do_sv_dump(pTHX_ I32 level, PerlIO *file, SV *sv, I32 nest, I32 maxnest,
                                                          UNI_DISPLAY_QQ));
 			if (HvEITER_get(hv) == he)
 			    PerlIO_printf(file, "[CURRENT] ");
-                        PerlIO_printf(file, "HASH = 0x%" UVxf "\n", (UV) hash);
+                        /*PerlIO_printf(file, "HASH = 0x%" UVxf "\n", (UV) hash);*/
                         do_sv_dump(level+1, file, elt, nest+1, maxnest, dumpops, pvlim);
                     }
 		}
@@ -3268,11 +3268,13 @@ Perl_deb_hek(pTHX_ HEK* hek, SV* val)
     else if (HEK_IS_SVKEY(hek)) {
         SV * const tmp = newSVpvs_flags("", SVs_TEMP);
         SV* sv = *(SV**)HEK_KEY(hek);
-        PerlIO_printf(Perl_debug_log, " [0x%08x SV:\"%s\" ", (unsigned)HEK_HASH(hek),
+        U32 hash = HEK_HASH_calc(hek);
+        PerlIO_printf(Perl_debug_log, " [0x%08x SV:\"%s\" ", (unsigned)hash,
                       pretty_pv_escape( tmp, SvPVX_const(sv), SvCUR(sv), SvUTF8(sv)));
     } else {
         SV * const tmp = newSVpvs_flags("", SVs_TEMP);
-        PerlIO_printf(Perl_debug_log, " [0x%08x \"%s\" ", (unsigned)HEK_HASH(hek),
+        U32 hash = HEK_HASH_calc(hek);
+        PerlIO_printf(Perl_debug_log, " [0x%08x \"%s\" ", (unsigned)hash,
                       pretty_pv_escape( tmp, HEK_KEY(hek), HEK_LEN(hek), HEK_UTF8(hek)));
         if (HEK_FLAGS(hek) > 1)
             PerlIO_printf(Perl_debug_log, "0x%x ", HEK_FLAGS(hek));

embed.fnc

@@ -744,6 +744,7 @@ AbmdRp	|bool	|hv_exists	|NULLOK HV *hv|NN const char *key|I32 klen
 AbmdRp	|bool	|hv_exists_ent	|NULLOK HV *hv|NN SV *keysv|U32 hash
 Abmdp	|SV**	|hv_fetch	|NULLOK HV *hv|NN const char *key|I32 klen \
 				|I32 lval
+: hash ignored
 Abmdp	|HE*	|hv_fetch_ent	|NULLOK HV *hv|NN SV *keysv|I32 lval|U32 hash
 #if defined(USE_CPERL)
 Ap	|void*	|hv_common	|NULLOK HV *hv|NULLOK SV *keysv \
@@ -2129,6 +2130,7 @@ Abpd	|SSize_t|unpack_str	|NN const char *pat|NN const char *patend|NN const char
 				|I32 ocnt|U32 flags
 Apd	|SSize_t|unpackstring	|NN const char *pat|NN const char *patend|NN const char *s \
 				|NN const char *strend|U32 flags
+: hash ignored
 Ap	|void	|unsharepvn	|NULLOK const char* sv|I32 len|U32 hash
 : Used in gv.c, hv.c
 #if defined(USE_CPERL)
@@ -2525,10 +2527,12 @@ s	|void	|hsplit		|NN HV *hv|STRLEN const oldsize|STRLEN newsize
 s	|void	|hv_free_entries|NN HV *hv
 s	|SV*	|hv_free_ent_ret|NN HV *hv|NN HE *entry
 sR	|HE*	|new_he
+: hash ignored
 sanR	|HEK*	|save_hek_flags	|NN const char *str|I32 len|U32 hash|int flags
 sn	|void	|hv_magic_check	|NN HV *hv|NN bool *needs_copy|NN bool *needs_store
 s	|void	|unshare_hek_or_pvn|NULLOK const HEK* hek|NULLOK const char* str|I32 len|U32 hash
 #  if defined(USE_CPERL)
+: hash mandatory
 sR	|HEK*	|share_hek_flags|NN const char *str|I32 len|U32 hash|int flags
 #  else
 : a perl5 security risk

ext/B/B.xs

@@ -2362,6 +2362,15 @@ HeKEY(he)
 U32
 HeHASH(he)
 	B::HE he
+    PREINIT:
+	U32 hash = 0;
+        HEK *hek;
+    CODE:
+        hek = HeKEY_hek(he);
+        PERL_HASH(hash, HEK_KEY(hek), HEK_LEN(hek));
+	RETVAL = hash;
+    OUTPUT:
+        RETVAL
 
 I32
 HeKLEN(he)