`no_std` version of `chunks`

[araspik]

Is it not possible to re-implement chunks for no_std? One issue I can see is that multiple iterators might exist simultaneously, complicating things. The only solution I can see (there are probably others) involves writing a no_std into_chunks() function which returns an IntoIterator only, which can be iterated as normal. Iterator might be implementable on top for &mut NoStdChunks (or whatever). I'm working in a full no_std environment and this function would be really helpful. I haven't done enough iteration stuff in Rust to know exactly how you would implement this, but it seems viable. P.S: Please create an issue to convert as many not(no_std) functions to no_std as possible sometime. I'm willing to help.