Split onion.[ch] into onion{,_fast,_tap}.[ch]

I'm going to want a generic "onionskin" type and set of wrappers, and
for that, it will be helpful to isolate the different circuit creation
handshakes.  Now the original handshake is in onion_tap.[ch], the
CREATE_FAST handshake is in onion_fast.[ch], and onion.[ch] now
handles the onion queue.

This commit does nothing but move code and adjust header files.