msg::CreateFast and CreatedFast confusing accessor naming

These cells both have as content a Vec<u8> which is called handshake in the constructor, and in the internal field name.

But the accessor is called body (or, into_body). This is confusing, especially because of the Body trait. (While the value is the same, the contextual semantics are different.)

IMO these methods should be renamed handshake. (The old ones could be retained as deprecated.)