[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

definitions/objects/endpoint.def

@@ -0,0 +1,30 @@
+# Channels are objects that enable synchronous IPC of
+# arbitrary-sized messages.
+
+object endpoint : kobject {
+    uid c5882f24a4c03b7e
+
+    method create [constructor]
+
+    # Send a message on a channel. Blocks until the message
+    # is received.
+    method send {
+        param tag uint64
+        param data buffer
+    }
+
+    # Receieve a message on a channel. Blocks until a message
+    # is available.
+    method receive {
+        param tag uint64 [out]
+        param data buffer [out]
+    }
+
+    # Send a message on a channel and then await a new message.
+    # Equivalent to calling send and then recieve, as a single
+    # operation.
+    method sendrecv {
+        param tag uint64 [inout]
+        param data buffer [inout]
+    }
+}