Need to clean up `network::encodable` and `network::serialize`

[apoelstra]

I think in the next major release we should fix the module names for the consensus encodable/decodable traits. Specifically I propose removing the serialize, encodable and consensus_params modules

• network::encodable::VarInt -> consensus::encode::VarInt and expose it at the crate level
• network::encodable::VarInt -> consensus::encode::CheckedData
• network::consensus_params -> consensus::params
• network::consensus_params::ConsensusParams -> consensus::params::Params
• network::encodable::ConsensusEncodable -> consensus::encode::Encodable but expose it as consensus::Encodable
• network::encodable::ConsensusDecodable -> consensus::encode::Decodable but expose it as consensus::Decodable
• network::seriailize::Error -> consensus::encode::Error
• network::serialize::BitcoinHash -> util::hash::BitcoinHash and expose it at crate level
• network::serialize::RawEncoder -> consensus::encode::RawEncoder and expose it as consensus::RawEncoder
• network::serialize::RawDecoder -> consensus::encode::RawDecoder and expose it as consensus::RawDecoder
• network::serialize::SimpleEncoder -> consensus::encode::Encoder and expose it as consensus::Encoder
• network::serialize::SimpleDecoder -> consensus::encode::Decoder and expose it as consensus::Decoder

Finally, we should deprecate everything in network::socket and network::listener because low-level networking is out of scope for this library. That leaves only p2p network messages under network, which is good. (Though we should also rename/restructure the modules there - I'll open yet another issue about that.)

Unrelatedly, we should also delete nu_select from macros.rs because this is horribly bitrotted and I doubt it even compiles anymore.

[dongcarl]

I'll do this as my PSBT code needs to be refactored over this.

[dongcarl]

@apoelstra It seems like VersionMessage's constructor takes in a Socket, shall I split that parameter into the respective fields, or create a SocketInfo struct in lieu of the deleted Socket?

[dongcarl]

Also, if anyone can take a look at why compilation is failing for my WIP commit, it'd be much appreciated. I've very clearly imported the MerkleRoot trait but it doesn't compile...