Create separate derives for SequenceEncode/SequenceDecode

[npmccallum]

Currently, we can derive(Sequence). But this implies ownership of values. Where *Ref types are used, derive(Sequence) cannot be. Perhaps we could split Sequence into SequenceEncode/SequenceDecode? I'm open to other solutions.

[tarcieri]

What might make sense is to split the Sequence custom derive apart so you can independently derive the DecodeValue and EncodeValue traits

[npmccallum]

Something would also need to derive FixedTag. I think that's why Sequence was done as a single derive.

[tarcieri]

FixedTag impls are trivial to write by hand

[baloo]

Should be fixed by #1722 (if not my bad, please re-open)