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/include.am

@@ -53,6 +53,8 @@ src_or_libtor_a_SOURCES = \
 	src/or/networkstatus.c				\
 	src/or/nodelist.c				\
 	src/or/onion.c					\
+	src/or/onion_fast.c				\
+	src/or/onion_tap.c				\
 	src/or/transports.c				\
 	src/or/policies.c				\
 	src/or/reasons.c				\
@@ -132,7 +134,9 @@ ORHEADERS = \
 	src/or/nodelist.h				\
 	src/or/ntmain.h					\
 	src/or/onion.h					\
+	src/or/onion_fast.h				\
 	src/or/onion_ntor.h				\
+	src/or/onion_tap.h				\
 	src/or/or.h					\
 	src/or/transports.h				\
 	src/or/policies.h				\