Refactor matrix transform, positions pervasively

* Do mapping to positions right in kscan handler, and then
  surface positions throughout the API.

app/include/zmk/matrix_transform.h

@@ -0,0 +1,3 @@
+#pragma once
+
+u32_t zmk_matrix_transform_row_column_to_position(u32_t row, u32_t column);