The refactoring added `next_is` which takes a `&str` as parameter, while
we only use it with strings of lengths 2 and 3. Replacing this by 2
dedicated methods (which can be added to the trait interface and only
specialized if needed) removes almost all the overhead that was added by
`Input`.
Hiding character fetching behind this interface allows us to create more
specific implementations when is appropriate. For instance, an instance
of `Input` can be created for a `&str`, allowing for borrowing and more
efficient peeking and traversing than if we were to fetch characters one
at a time and placing them into a temporary buffer.
