Peekable::try_peek

[nazar-pc]

Peekable combinator is currenly less useful than it could have been.

I have a use case where it would be helpful to check the next element in a stream (if it is already there) and start preparation for its processing, but at the same time I don't want to delay processing of current element. So I need .try_peek() that is similar to .try_next(), but doesn't advance the stream itself.

[taiki-e]

Seems reasonable to me.