Implement RAMF serialisation and deserialisation as streams

[gnarea]

This is the memory-efficient alternative to the buffer-based implementation in #2 and #8. This will typically be useful when processing cargo, not parcels.

See potential encryption/decryption implementation: https://lollyrock.com/posts/nodejs-encryption/. We may want to use Scramjet's BufferStream to help with this.

This depends on AwalaApp/specs#14

See also: PeculiarVentures/PKI.js#237