This patch improves functions my_xxx_to_str() using the idea
introduced in this change in MySQL-8.0:

  commit 8d10f2fff6bbdea7f436b868ebb5fd811defc68a
  Author: Knut Anders Hatlen <knut.hatlen@oracle.com>
  Date:   Thu Oct 10 13:55:07 2019 +0200
    Bug#30472888: IMPROVE THE PERFORMANCE OF INTEGER HANDLING IN THE TEXT PROTOCOL

The new way prints 2 digits at a time and demonstrates a very impressing query time reduce:
10% to 38%, depending on the exact data type and the number of fractional digits:

  SELECT BENCHMARK(10*1000*1000,CONCAT(TIME'10:20:30'));
  SELECT BENCHMARK(10*1000*1000,CONCAT(TIME'10:20:30.123456'));
  SELECT BENCHMARK(10*1000*1000,CONCAT(DATE'2001-01-01'));
  SELECT BENCHMARK(10*1000*1000,CONCAT(TIMESTAMP'2001-01-01 10:20:30'));
  SELECT BENCHMARK(10*1000*1000,CONCAT(TIMESTAMP'2001-01-01 10:20:30.123456'));

See MDEV for details on the benchmark results.