BASE 62 uses 0-9, A-Z and then a-z to give the numbers 0-61. This patch
increases the range of the string functions to cover this.

Based on ideas and tests in PR #2589, but re-written into the charset
functions.

Includes fix by Sergei, UBSAN complained:
ctype-simple.c:683:38: runtime error: negation of -9223372036854775808
cannot be represented in type 'long long int'; cast to an unsigned
type to negate this value to itself
Co-authored-by: Weijun Huang <huangweijun1001@gmail.com>
Co-authored-by: Sergei Golubchik <serg@mariadb.org>