Kernel: static vs non-static constexpr variables

Problem:
- `static` variables consume memory and sometimes are less
  optimizable.
- `static const` variables can be `constexpr`, usually.
- `static` function-local variables require an initialization check
  every time the function is run.

Solution:
- If a global `static` variable is only used in a single function then
  move it into the function and make it non-`static` and `constexpr`.
- Make all global `static` variables `constexpr` instead of `const`.
- Change function-local `static const[expr]` variables to be just
  `constexpr`.

Kernel/Devices/HID/HIDManagement.h

@@ -60,7 +60,7 @@ private:
 
 class KeyboardClient {
 public:
-    virtual ~KeyboardClient();
+    virtual ~KeyboardClient() = default;
     virtual void on_key_pressed(KeyEvent) = 0;
 };