Significantly overhaul OPT code

Simplify the mess that was option setting (2 redundant variables !?)
Move options to a separate file
Have "modules" own their options, and OPT only access them (less redundancy)
Simplify code, respect naming conventions better

include/asm/opt.h

@@ -0,0 +1,22 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2021, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_OPT_H
+#define RGBDS_OPT_H
+
+
+void opt_B(char chars[2]);
+void opt_G(char chars[4]);
+void opt_P(uint8_t fill);
+void opt_Parse(char const *option);
+
+void opt_Push(void);
+void opt_Pop(void);
+
+
+#endif