[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/definitions/templates/syscall/client/private/syscall_trampolines.s

@@ -0,0 +1,23 @@
+%macro SYSCALL 2
+	global __syscall_%1
+	__syscall_%1:
+		push rbp
+		mov rbp, rsp
+
+		; args should already be in rdi, etc, but rcx will
+		; get stomped, so stash it in r10, which isn't a
+		; callee-saved register, but also isn't used in the
+		; function call ABI.
+		mov r10, rcx
+
+		mov rax, %2
+		syscall
+		; result is now already in rax, so just return
+
+		pop rbp
+		ret
+%endmacro
+
+{% for id, scope, method in interface.methods %}
+SYSCALL {% if scope %}{{ scope.name }}_{% endif %}{{ method.name }} {{ id }}
+{% endfor %}