[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

src/kernel/syscalls.inc.cog

@@ -0,0 +1,11 @@
+; vim: ft=asm
+; [[[cog code generation
+; from definitions.context import Context
+;
+; ctx = Context(definitions_path)
+; ctx.parse("syscalls.def")
+; syscalls = ctx.interfaces['syscalls']
+; cog.outl(f"NUM_SYSCALLS equ {len(syscalls.methods)}")
+; ]]]
+; [[[end]]]
+