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.

src/or/cpuworker.c

@@ -23,6 +23,7 @@
 #include "cpuworker.h"
 #include "main.h"
 #include "onion.h"
+#include "onion_tap.h"
 #include "router.h"
 
 /** The maximum number of cpuworker processes we will keep around. */