Make RGBLINK able to link SDCC object files

This requires a LOT of tricky code, mostly due to the format itself being,
er, not the most straightforward.
Everything is converted to existing RGBLINK concepts (sections, patches,
etc.), so the core code is essentially unchanged.
(A couple of genuine RGBLINK bugs were uncovered along the way, so some of
the core code *is* changed, notably regarding `SECTION FRAGMENT`s.)

All of this code was clean-roomed, so SDCC's GPLv2 license does not apply.

include/link/sdas_obj.h

@@ -0,0 +1,19 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/* Assigning all sections a place */
+#ifndef RGBDS_LINK_SDAS_OBJ_H
+#define RGBDS_LINK_SDAS_OBJ_H
+
+#include <stdio.h>
+
+struct FileStackNode;
+
+void sdobj_ReadFile(struct FileStackNode const *fileName, FILE *file);
+
+#endif /* RGBDS_LINK_SDAS_OBJ_H */