Add support for multiple charmaps

This adds two new directives: newcharmap and setcharmap.
newcharmap creates a new charmap and switches to it.
setcharmap switches to an existing charmap.

include/asm/util.h

@@ -0,0 +1,17 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 1997-2019, Carsten Sorensen and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_UTIL_H
+#define RGBDS_UTIL_H
+
+#include <stdint.h>
+
+uint32_t calchash(const char *s);
+int32_t readUTF8Char(char *dest, char *src);
+
+#endif /* RGBDS_UTIL_H */