[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

scripts/idgen

@@ -0,0 +1,11 @@
+#!/usr/bin/env python3
+
+def hashid(s):
+    from hashlib import shake_128 as sh
+    return sh(s.encode('utf-8')).hexdigest(8)
+
+import sys
+
+for arg in sys.argv[1:]:
+    id = hashid(arg)
+    print(f"{arg}: {id}")