Made the keymap a constexpr array

This enables compile time exploration of the keymap
from all code that is part of the sketch's compilation unit.

Signed-off-by: Florian Fleissner <florian.fleissner@inpartik.de>

src/kaleidoscope/layers.cpp

@@ -33,7 +33,7 @@ __attribute__((weak))
 uint8_t layer_count = 0;
 
 __attribute__((weak))
-extern const Key keymaps_linear[][kaleidoscope_internal::device.matrix_rows * kaleidoscope_internal::device.matrix_columns] = {};
+extern constexpr Key keymaps_linear[][kaleidoscope_internal::device.matrix_rows * kaleidoscope_internal::device.matrix_columns] = {};
 
 namespace kaleidoscope {
 uint32_t Layer_::layer_state_;

src/kaleidoscope/layers.h

@@ -26,7 +26,7 @@
 // file (*.ino) to define the keymap[] array that holds the user's
 // layers. It also computes the number of layers in that keymap.
 #define KEYMAPS(layers...)				                                    __NL__ \
-  const Key keymaps_linear[][kaleidoscope_internal::device.matrix_rows * kaleidoscope_internal::device.matrix_columns] PROGMEM = { layers };           __NL__ \
+  constexpr Key keymaps_linear[][kaleidoscope_internal::device.matrix_rows * kaleidoscope_internal::device.matrix_columns] PROGMEM = { layers };           __NL__ \
   uint8_t layer_count                                                   __NL__ \
      = sizeof(keymaps_linear) / sizeof(*keymaps_linear);                __NL__ \
                                                                         __NL__ \