[kernel] Pass objects not handles to syscall impls

This commit contains a couple large, interdependent changes:

- In preparation for capability checking, the _syscall_verify_*
  functions now load most handles passed in, and verify that they exist
  and are of the correct type. Lists and out-handles are not converted
  to objects.
- Also in preparation for capability checking, the internal
  representation of handles has changed. j6_handle_t is now 32 bits, and
  a new j6_cap_t (also 32 bits) is added. Handles of a process are now a
  util::map<j6_handle_t, handle> where handle is a new struct containing
  the id, capabilities, and object pointer.
- The kernel object definition DSL gained a few changes to support auto
  generating the handle -> object conversion in the _syscall_verify_*
  functions, mostly knowing the object type, and an optional "cname"
  attribute on objects where their names differ from C++ code.
  (Specifically vma/vm_area)
- Kernel object code and other code under kernel/objects is now in a new
  obj:: namespace, because fuck you <cstdlib> for putting "system" in
  the global namespace. Why even have that header then?
- Kernel object types constructed with the construct_handle helper now
  have a creation_caps static member to declare what capabilities a
  newly created object's handle should have.

src/kernel/cpu.h

@@ -5,11 +5,14 @@
 class GDT;
 class IDT;
 class lapic;
-class process;
 struct TCB;
-class thread;
 class TSS;
 
+namespace obj {
+    class process;
+    class thread;
+}
+
 struct cpu_state
 {
     uint64_t r15, r14, r13, r12, r11, r10, r9,  r8;
@@ -47,8 +50,8 @@ struct cpu_data
     uintptr_t rsp3;
     uint64_t rflags3;
     TCB *tcb;
-    thread *thread;
-    process *process;
+    obj::thread *thread;
+    obj::process *process;
     IDT *idt;
     TSS *tss;
     GDT *gdt;