Add reference implementation for ntor, plus compatibility test

Before I started coding ntor in C, I did another one in Python.
Turns out, they interoperate just fine.

src/test/include.am

@@ -53,3 +53,16 @@ src_test_bench_LDADD = src/or/libtor.a src/common/libor.a \
 noinst_HEADERS+= \
 	src/test/test.h
 
+if CURVE25519_ENABLED
+noinst_PROGRAMS+= src/test/test-ntor-cl
+src_test_test_ntor_cl_SOURCES  = src/test/test_ntor_cl.c
+src_test_test_ntor_cl_LDFLAGS = @TOR_LDFLAGS_zlib@ @TOR_LDFLAGS_openssl@
+src_test_test_ntor_cl_LDADD = src/or/libtor.a src/common/libor.a \
+	src/common/libor-crypto.a $(LIBDONNA) \
+	@TOR_ZLIB_LIBS@ @TOR_LIB_MATH@ \
+	@TOR_OPENSSL_LIBS@ @TOR_LIB_WS32@ @TOR_LIB_GDI@
+src_test_test_ntor_cl_AM_CPPFLAGS =	       \
+	-I"$(top_srcdir)/src/or"
+
+endif
+