Add some Rust utility functions and print support

This gives an indication in the log that Tor was built with Rust
support, as well as laying some groundwork for further string-returning
APIs to be converted to Rust

src/common/include.am

@@ -100,6 +100,11 @@ LIBOR_A_SRC = \
   $(threads_impl_source)				\
   $(readpassphrase_source)
 
+if USE_RUST
+else
+LIBOR_A_SRC += src/common/compat_rust.c
+endif
+
 src/common/src_common_libor_testing_a-log.$(OBJEXT) \
   src/common/log.$(OBJEXT): micro-revision.i
 
@@ -146,6 +151,7 @@ COMMONHEADERS = \
   src/common/compat.h				\
   src/common/compat_libevent.h			\
   src/common/compat_openssl.h			\
+  src/common/compat_rust.h			\
   src/common/compat_threads.h			\
   src/common/compat_time.h			\
   src/common/compress.h			\