kernel: Eliminate kernel global state

As means to pave the way for getting rid of global state within core,
This eliminates kernel global state by removing all globals. Instead
this introduces a KernelCore class which acts as a kernel instance. This
instance lives in the System class, which keeps its lifetime contained
to the lifetime of the System class.

This also forces the kernel types to actually interact with the main
kernel instance itself instead of having transient kernel state placed
all over several translation units, keeping everything together. It also
has a nice consequence of making dependencies much more explicit.

This also makes our initialization a tad bit more correct. Previously we
were creating a kernel process before the actual kernel was initialized,
which doesn't really make much sense.

The KernelCore class itself follows the PImpl idiom, which allows
keeping all the implementation details sealed away from everything else,
which forces the use of the exposed API and allows us to avoid any
unnecessary inclusions within the main kernel header.

src/core/core.h

@@ -12,6 +12,7 @@
 #include "common/common_types.h"
 #include "core/arm/exclusive_monitor.h"
 #include "core/core_cpu.h"
+#include "core/hle/kernel/kernel.h"
 #include "core/hle/kernel/object.h"
 #include "core/hle/kernel/scheduler.h"
 #include "core/loader/loader.h"
@@ -188,6 +189,12 @@ public:
         return current_process;
     }
 
+    /// Provides a reference to the kernel instance.
+    Kernel::KernelCore& Kernel();
+
+    /// Provides a constant reference to the kernel instance.
+    const Kernel::KernelCore& Kernel() const;
+
     /// Gets the name of the current game
     Loader::ResultStatus GetGameName(std::string& out) const {
         if (app_loader == nullptr)
@@ -246,6 +253,7 @@ private:
      */
     ResultStatus Init(Frontend::EmuWindow& emu_window);
 
+    Kernel::KernelCore kernel;
     /// RealVfsFilesystem instance
     FileSys::VirtualFilesystem virtual_filesystem;
     /// AppLoader used to load the current executing application