move client request handler into core::http1::client

[jkarneges]

This moves the HTTP client code from src/connection.rs into a new module next to the new server module. With the new module naming, connection::ClientRequest becomes core::http1::client::Request. Unlike the new server module, this client module is actually used. This PR removes the code from src/connection.rs, adds the new module, and updates all references to refer to the new module. Only very minor changes were made along the way, basically enough to make it work in the new location (renaming types, marking types/fns as pub, wrapping field access with methods, etc).