ecdsa: der: use impl EncodeValue/DecodeValue on SignatureRef

[dishmaker]

This way we avoid using internals of der:

• der::SliceWriter
• der::SliceReader
• tag.assert_eq
• writer.sequence

[tarcieri]

Those aren't really "internals": they're part of the public API, though this is perhaps more really idiomatic with how the rest of the library works.

Notably this doesn't really reduce code size.