AppendByteOrder specifies new methods for LittleEndian and BigEndian
for appending an unsigned integer to a byte slice.
The performance of AppendXXX methods are slower than PutXXX methods
since the former needs to do a few slice operations,
while the latter is essentially a single integer store.
In practice, existing usages of PutXXX performed slicing operations
around the call such that this cost was present, regardless.