Use new_checked by default

[dlrobertson]

Currently all Packet types have at least the two constructor methods new and new_checked. new does no length checks, while new_checked obviously does. AFAIK in std etc typically the "safe" version is the default and the "unsafe" version is explicitly called with <name>_unchecked. Would it make sense to do something like the following?

1. deprecate Packet::new_checked
2. make Packet::new functions do what Packet::new_checked does
3. add Packet::new_unchecked functions that do what Packet::new does

[dlrobertson]

Just a question/thought

[whitequark]

There is nothing inherently safer about constructing packets with new_checked because you can still get panics if you modify a length field inside the packet and then use an accessor.

Moreover, it's not even that one should be preferred over the other. In a typical network stack that both sends and receives packets there would be approximately as many calls to new as to new_checked.

I'm open to other suggestions for these names, but I think that safety is the wrong angle here.