[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/syscalls.def

@@ -0,0 +1,27 @@
+import "objects/system.def"
+import "objects/kobject.def"
+import "objects/process.def"
+import "objects/thread.def"
+import "objects/channel.def"
+import "objects/endpoint.def"
+import "objects/vma.def"
+
+interface syscalls [syscall] {
+    uid 01d9b6a948961097
+
+    expose object system
+    expose object kobject
+    expose object process
+    expose object thread
+    expose object channel
+    expose object endpoint
+    expose object vma
+
+    # Simple no-op syscall for testing
+    function noop
+
+    # Write a message to the kernel log
+    function log {
+        param message string
+    }
+}