core: Migrate current_process pointer to the kernel

Given we now have the kernel as a class, it doesn't make sense to keep
the current process pointer within the System class, as processes are
related to the kernel.

This also gets rid of a subtle case where memory wouldn't be freed on
core shutdown, as the current_process pointer would never be reset,
causing the pointed to contents to continue to live.

src/core/hle/kernel/kernel.h

@@ -65,6 +65,15 @@ public:
     /// Adds the given shared pointer to an internal list of active processes.
     void AppendNewProcess(SharedPtr<Process> process);
 
+    /// Makes the given process the new current process.
+    void MakeCurrentProcess(SharedPtr<Process> process);
+
+    /// Retrieves a reference to the current process.
+    SharedPtr<Process>& CurrentProcess();
+
+    /// Retrieves a const reference to the current process.
+    const SharedPtr<Process>& CurrentProcess() const;
+
     /// Adds a port to the named port table
     void AddNamedPort(std::string name, SharedPtr<ClientPort> port);