Add `KeyAddr::none()` constant & `KeyAddr::clear()` method

This is a convenience only, but it's much more straightforward than the
expression `key_addr = KeyAddr(KeyAddr::invalide_state)` or the overly enigmatic
`key_addr = KeyAddr()`.

`KeyAddr::none()` is meant to be useful for initialization of variables:
`KeyAddr my_addr = KeyAddr::none()`

`KeyAddr::clear()` is meant to be the counterpart of the `isValid()` test, to
set an existing variable to an invalid address:
`my_addr.clear()` vs `if (my_addr.isValid()) ...`

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

src/kaleidoscope/MatrixAddr.h

@@ -1,5 +1,5 @@
 /* Kaleidoscope - Firmware for computer input devices
- * Copyright (C) 2013-2019  Keyboard.io, Inc.
+ * Copyright (C) 2013-2020  Keyboard.io, Inc.
  *
  * This program is free software: you can redistribute it and/or modify it under
  * the terms of the GNU General Public License as published by the Free Software
@@ -74,6 +74,18 @@ class MatrixAddr {
                   "Matrix type conversion failed. Source type must not have greater col size than target type");
   }
 
+  // Get an invalid key address. Useful when initializing a state variable or
+  // function parameter that handles invalid addresses.
+  static constexpr ThisType none() {
+    return ThisType(invalid_state);
+  }
+
+  // Invalidate the key address. Useful in plugins that need a state variable
+  // that is sometimes valid, and sometimes not.
+  void clear() {
+    offset_ = invalid_state;
+  }
+
   constexpr uint8_t row() const {
     return offset_ / cols;
   }