[stdlib] Move StringRef find() implementation to Span

[martinvuyk]

Move StringRef find() implementation to Span

[JoeLoser]

Question It seems a bit weird from an API design perspective to have this "find" function in span, can you help me understand why we'd want this? It feels more coupled to string algorithms, and Span isn't just for a view over string data.

[martinvuyk]

It can be used if you want to find occurrences of a specific scalar value sequence. My main thought was that we'll lose the ability to do that once StringSlice.find() is fixed to use unicode codepoints and it introduces quite some overhead for algorithms that are faster using raw bytes. We could also make this private WDYT?

Also, once PR's #3577 DType.get_dtype() gets merged, List.index() can also delegate to this

[martinvuyk]

@JoeLoser gentle ping and also to add something, since it seems like we are moving toward unifying Python's bytes into Span[UInt8]in PR #3636, request #3634 and many other places. Python's bytes.find is actually a method, and many other things that we put in StringSlice and StringRef are actually operations from bytes.

[msaelices]

I think this is a great change for overall performance when manipulating slices. It opens a lot of possibilities.