Removed bounds on Append, PluckTail, Prepend, and Pluck.

[thorjelly]

While I completely agree with the stated goal of making <() as Append<T>>::Output = (T,), I feel PR #6 introduced overly restrictive and unnecessary bounds.

This has come up for me because I found it necessary to implement my own Empty types, and I still want to be able to append them and output a tuple. But the bound on Append::Output makes it impossible to implement that, because I cannot implement PluckTail a second time on (T,).

I am unsure why these bounds were necessary for the stated goal. ().append(x).pluck_tail() is completely backwards compatible without them with the other changes in PR #6. I understand that with a custom empty type it wouldn't be (the sequence would return () instead of the original custom empty), but I find that less of a limitation than not allowing a custom implementation at all, so I feel removing them makes the library more useful.