[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/mailbox.def

@@ -0,0 +1,33 @@
+# Mailboxes are objects that enable asynchronous IPC via event notification.
+# This is a second line of documentation
+
+object mailbox {
+    uid 99934ad04ece1e07
+
+    # Create an unbound mailbox
+    method create [constructor]
+
+    method close [destructor]
+
+    method bind {
+        param index uint
+        param source object kobject
+        param event uint
+    }
+
+    method unbind {
+        param index uint
+    }
+
+    method notify {
+        param index uint
+    }
+
+    method wait {
+        param bitmap uint64 [out]
+    }
+
+    method ack {
+        param bitmap uint64
+    }
+}