[kernel] Create process kernel object

Re-implent the concept of processes as separate from threads, and as a
kobject API object. Also improve scheduler::prune which was doing some
unnecessary iterations.

modules.yaml

@@ -40,6 +40,7 @@ modules:
             - src/kernel/objects/handle.cpp
             - src/kernel/objects/kobject.cpp
             - src/kernel/objects/thread.cpp
+            - src/kernel/objects/process.cpp
             - src/kernel/page_manager.cpp
             - src/kernel/pci.cpp
             - src/kernel/scheduler.cpp