Skip validity and length calls when the byte cannot be part of a multibyte sequence

Almost every character set can be given a number N such that a multibyte
sequence starts with a byte higher than that number N. This allows us to
skip a lot of work. To ensure the correctness of this, a sanity check is
implemented that exhaustively tries every 4-byte sequence for every
character set and checks for consistency issues.

This finally gives:
  Time (mean ± σ):     120.2 ms ±   1.2 ms    [User: 116.9 ms, System: 2.8 ms]
  Range (min … max):   118.0 ms … 122.9 ms    24 runs

Author: nielsdos

ext/mysqlnd/mysqlnd.h

@@ -45,6 +45,8 @@
 #define MYSQLND_DBG_ENABLED 0
 #endif
 
+#define MYSQLND_CHARSETS_SANITY_CHECK 0
+
 #ifdef ZTS
 #include "TSRM.h"
 #endif