`Buf::try_advance` support

`Buf::advance` is a required method and panics if it runs out of bytes. It would be useful to have a similar functionality which doesn't panic, returning a `TryGetError`.

`Buf::try_advance` could be the required method instead of `Buf::advance`, with the latter being a wrapper on top of the former which just unwraps and panics. This may be undesirable as it would break all implementations!