ext/standard: Optimize single-character searches in strcspn() and strpbrk()

[LamentXU123]

When I was optimizing str_repeat last month there is a logic inside it:

php-src/ext/standard/string.c

Lines 5648 to 5651 in fab508f

	/* Heavy optimization for situations where input string is 1 byte long */
	if (ZSTR_LEN(input_str) == 1) {
	memset(ZSTR_VAL(result), *ZSTR_VAL(input_str), mult);
	} else {

Today when reading this piece of code I realize same stuff can be applied on these two functions.

[LamentXU123]

Thanks!