prefer mem* funcs instead of str*

wrappers expect length, thus no need to look for '\0'

only missing piece is case-insensitive memcmp (memicmp?)
since implementation requires it, manually do the tolower() loop

update replace() w/o '\0' assumptions, always use mem ranges
drop strstr usage and reuse loop blocks

also returns actual len diff after comparison is otherwise equal

Author: mcspr