ext/mbstring: Optimize valid UTF-8 in mb_substr()

[kamil-tekiela]

This PR adds a fast path for valid UTF-8 strings in mb_substr. It skips bytes in 256 chunks. The performance improvement is noticeable with longer strings and only ones that are flagged as valid UTF-8.

@alexdowad What do you think? Is this worth doing? I haven't got a fuzzer, but I don't anticipate any behavioural changes.

[alexdowad]

@kamil-tekiela Fuzzing is 100% necessary. Otherwise, it is very, very easy for subtle bugs to slip through.

It's good that you optimized c < 0x80 || (c & 0xC0) != 0x80 to just (c & 0xC0) != 0x80; I think that change alone can go direct to master.

[kamil-tekiela]

Do you have a fuzzer available? If not can you give me hints on how to build one?

[alexdowad]

@kamil-tekiela When I have some time, I could fuzz this for you. Or, if you want to do it yourself, that might be a very good idea... fuzzing is an extremely powerful technique for finding bugs, and is an awesome tool for any programmer to know how to use.

I once wrote up my workflow for fuzzing new code in mbstring here: #10828 (comment)

[kamil-tekiela]

I tried fuzzing as you said and it didn't find anything. Not sure I've done it properly, but when I intentionally introduced a bug it found it immediately.