der_derive: add DecodeValue, EncodeValue macros

[dishmaker]

Fixes

• Create separate derives for SequenceEncode/SequenceDecode #1705

Adds DecodeValue macro:

#[derive(DecodeValue)]structMyDecodeOnly{field:u8,}implFixedTagforMyDecodeOnly{constTAG:Tag = Tag::Sequence;}

and EncodeValue:

#[derive(EncodeValue)]structMyEncodeOnly{field:u8,}implFixedTagforMyEncodeOnly{constTAG:Tag = Tag::Sequence;}

[tarcieri]

There aren't corresponding traits for these (and I don't think there should be).

Instead I would suggest making it possible to derive DecodeValue and EncodeValue

[baloo]

Thanks a lot!