Work on the filewatch module.

Currently a wrapper around inotify, should be expanded to work with
kqueue, icop, and eventually a implementation with polling and stat.

src/core/util.h

@@ -204,6 +204,7 @@ extern const JanetAbstractType janet_address_type;
 void janet_lib_ev(JanetTable *env);
 void janet_ev_mark(void);
 int janet_make_pipe(JanetHandle handles[2], int mode);
+void janet_lib_filewatch(JanetTable *env);
 #endif
 #ifdef JANET_FFI
 void janet_lib_ffi(JanetTable *env);