Message encoding functions have been renamed to catch any uses.
The switch to the new encoder can cause subtle incompatibilities.
If there are any users outside of our tree, they will at least be
alerted that there was a change.
NewMsg no longer exists. The replacements for EncodeMsg are called
Send and SendItems.
This mostly changes how information is passed around.
Instead of using many function parameters and return values,
put the entire state in a struct and pass that.
This also adds back derivation of ecdhe-shared-secret. I deleted
it by accident in a previous refactoring.
The diff is a bit bigger than expected because the protocol handshake
logic has moved out of Peer. This is necessary because the protocol
handshake will have custom framing in the final protocol.
