[project] Generate syscalls from new interface DSL

This change adds a new interface DSL for specifying objects (with
methods) and interfaces (that expose objects, and optionally have their
own methods).

Significant changes:

- Add the new scripts/definitions Python module to parse the DSL
- Add the new definitions directory containing DSL definition files
- Use cog to generate syscall-related code in kernel and libj6
- Unify ordering of pointer + length pairs in interfaces

src/kernel/objects/endpoint.h

@@ -27,10 +27,10 @@ public:
     /// Send a message to a thread waiting to receive on this endpoint. If no threads
     /// are currently trying to receive, block the current thread.
     /// \arg tag   The application-specified message tag
-    /// \arg len   The size in bytes of the message
     /// \arg data  The message data
+    /// \arg len   The size in bytes of the message
     /// \returns   j6_status_ok on success
-    j6_status_t send(j6_tag_t tag, size_t len, void *data);
+    j6_status_t send(j6_tag_t tag, const void *data, size_t data_len);
 
     /// Receive a message from a thread waiting to send on this endpoint. If no threads
     /// are currently trying to send, block the current thread.
@@ -38,7 +38,7 @@ public:
     /// \arg len   [in] The size in bytes of the buffer [out] Number of bytes in the message
     /// \arg data  Buffer for copying message data into
     /// \returns   j6_status_ok on success
-    j6_status_t receive(j6_tag_t *tag, size_t *len, void *data);
+    j6_status_t receive(j6_tag_t *tag, void *data, size_t *data_len);
 
     /// Give the listener on the endpoint a message that a bound IRQ has been signalled
     /// \arg irq  The IRQ that caused this signal
@@ -48,7 +48,10 @@ private:
     struct thread_data
     {
         thread *th;
-        void *data;
+        union {
+            const void *data;
+            void *buffer;
+        };
         union {
             j6_tag_t *tag_p;
             j6_tag_t tag;