dsa: reduce `Box<[u8]>` allocation in `SignatureEncoding::to_vec`

[dishmaker]

Rewritten as SignatureRef

[tarcieri]

The crypto-bigint crate has native support for der, although it's missing DecodeValue/EncodeValue impls for BoxedUint (and NonZero<BoxedUint>). They exist for Uint.

Perhaps support could be added there so there's not need for complex encoding logic in the dsa crate.

[dishmaker]

Depends on:

• https://github.com/RustCrypto/crypto-bigint/pull/851

[tarcieri]

Closing as stale