hash.c, string.c: improve hash function quality

Use FNV-1a (xor-then-multiply) instead of FNV-1 for better avalanche
in byte hashing. Strengthen the hash finalizer in mrb_obj_hash_code()
with multiply-xorshift to improve distribution for integer and symbol
keys with power-of-two table sizes.

Co-authored-by: Claude <noreply@anthropic.com>

src/hash.c

@@ -361,7 +361,10 @@ mrb_obj_hash_code(mrb_state *mrb, mrb_value key)
     hash_code = U32(tt) ^ U32(mrb_integer(hash_code_obj));
     break;
   }
-  return hash_code ^ (hash_code << 2) ^ (hash_code >> 2);
+  hash_code ^= hash_code >> 16;
+  hash_code *= 0x45d9f3b;
+  hash_code ^= hash_code >> 16;
+  return hash_code;
 }
 
 static uint32_t

src/string.c

@@ -2014,18 +2014,18 @@ mrb_byte_hash_step(const uint8_t *s, mrb_int len, uint32_t hval)
   const uint8_t *send = s + len;
 
   /*
-   * FNV-1 hash each octet in the buffer
+   * FNV-1a hash each octet in the buffer
    */
   while (s < send) {
+    /* xor the bottom with the current octet */
+    hval ^= (uint32_t)*s++;
+
     /* multiply by the 32-bit FNV magic prime mod 2^32 */
 #if defined(NO_FNV_GCC_OPTIMIZATION)
     hval *= FNV_32_PRIME;
 #else
     hval += (hval<<1) + (hval<<4) + (hval<<7) + (hval<<8) + (hval<<24);
 #endif
-
-    /* xor the bottom with the current octet */
-    hval ^= (uint32_t)*s++;
   }
 
   /* return our new hash value */