[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/syscalls/channel.cpp

@@ -4,29 +4,27 @@
 #include "objects/channel.h"
 #include "syscalls/helpers.h"
 
+using namespace obj;
+
 namespace syscalls {
 
 j6_status_t
-channel_create(j6_handle_t *handle)
+channel_create(j6_handle_t *self)
 {
-    construct_handle<channel>(handle);
+    construct_handle<channel>(self);
     return j6_status_ok;
 }
 
 j6_status_t
-channel_send(j6_handle_t handle, size_t *len, void *data)
+channel_send(channel *self, size_t *len, void *data)
 {
-    channel *c = get_handle<channel>(handle);
-    if (!c) return j6_err_invalid_arg;
-    return c->enqueue(len, data);
+    return self->enqueue(len, data);
 }
 
 j6_status_t
-channel_receive(j6_handle_t handle, size_t *len, void *data)
+channel_receive(channel *self, size_t *len, void *data)
 {
-    channel *c = get_handle<channel>(handle);
-    if (!c) return j6_err_invalid_arg;
-    return c->dequeue(len, data);
+    return self->dequeue(len, data);
 }
 
 } // namespace syscalls