`Buf::try_advance` support

[Tom01098]

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!