Add JANET_MARSH_UNSAFE flag.

This allows unmarshal to optional marshal raw
pointers and cfunctions and send them across threads.
This flag is only exposed in the C API as it is very easy
to misuse and cause segfaults.

src/include/janet.h

@@ -1310,6 +1310,8 @@ typedef JanetBuildConfig(*JanetModconf)(void);
 JANET_API JanetModule janet_native(const char *name, JanetString *error);
 
 /* Marshaling */
+#define JANET_MARSHAL_UNSAFE 0x20000
+
 JANET_API void janet_marshal(
     JanetBuffer *buf,
     Janet x,