Competing with C in a simple loop

[Christopher Done]

Purely as an experiment, I've written a function that uses ByteString to
simply elemIndex it's way across a string here. Look for <, then look for
>. Repeat until done.

https://github.com/chrisdone/xeno (under src/Xeno.hs)

But if you scroll down the README to the 182kb file example, you see that
hexml takes 33us and xeno takes 111us. That's surprising to me because I'm
doing just a walk across a string and hexml is doing a full parse. It's
written in C, but still, 3x faster AND doing allocations and more work.

I tried replacing the ByteString with a raw Ptr Word8 and it didn't make a
difference, actually increased time a little bit.

My weigh results indicate that it's not doing any allocations during the
process, at least nothing linear or above. So, I haven't looked at the core
or asm yet, but I'm guessing it's simply doing more instructions and/or
indirections than necessary.

You can reproduce this with stack build --bench xeno

Can anyone make an improvement to the speed? I already nerd sniped myself
enough with this, so I'm spreading the bug elsewhere. I think it's a pretty
good "raw" performance exercise and possibly something that could serve as
a tutorial on haskell performance.

Ciao!
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