DataView and BufReader should either default to LittleEndian or require explicit endianness

It is way too easy to forget the annotations, and they are incompatible with some forms of the methods. Since LittleEndian is almost always what we want and NativeEndian is almost never, it seems like LittleEndian is the better implict default. Either way, I think we should never implicitly default to Native and require making it explicit.