Implement method `CppStringT::startswith()`

[schmouk]

"str.startswith(prefix[, start[, end]])

Return True if string starts with the prefix, otherwise return False. prefix can also be a tuple of prefixes to look for. With optional start, test string beginning at that position. With optional end, stop comparing string at that position."

[schmouk]

Completed.
Optimization done also on formerly implemented method endswith().