Break files apart into more modules

Break files apart into more modules

 * \file circuitbuild.c
 * \brief The actual details of building circuits.

 * \file circuitlist.c
 * \brief Manage the global circuit list.

 * \file circuituse.c
 * \brief Launch the right sort of circuits, attach streams to them.

 * \file connection_edge.c
 * \brief Handle edge streams.

 * \file onion.c
 * \brief Functions to queue create cells, and handle onionskin
 * parsing and creation.

 * \file relay.c
 * \brief Handle relay cell encryption/decryption, plus packaging and
 * receiving from circuits.

svn:r1863