der_derive: add DecodeValue, EncodeValue macros

[dishmaker]

Fixes

• Create separate derives for SequenceEncode/SequenceDecode #1705

Adds DecodeValue macro:

#[derive(DecodeValue)]
struct MyDecodeOnly {
    field: u8,
}
impl FixedTag for MyDecodeOnly {
    const TAG: Tag = Tag::Sequence;
}

and EncodeValue:

#[derive(EncodeValue)]
struct MyEncodeOnly {
    field: u8,
}
impl FixedTag for MyEncodeOnly {
    const TAG: 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