[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/thread.def

@@ -0,0 +1,18 @@
+object thread : kobject {
+    uid 11f23e593d5761bd
+
+    method create [constructor] {
+        param entrypoint address
+    }
+
+    method kill [destructor]
+
+    method exit [static] {
+        param status int32
+    }
+
+    method pause [static]
+    method sleep [static] {
+        param until uint64
+    }
+}