"is empty" for byte sequence

[TimothyGu]

Byte sequence isn't defined in terms of a list, so it needs its own definition for "is empty":

A byte sequence is empty if its length is zero.

[annevk]

See #91. This might change a bit.

[annevk]

Also, can't you just compare with the empty byte sequence? That's what we do for strings (which I consider roughly equivalent) all the time.