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.

src/asm/util.c

@@ -0,0 +1,46 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 1997-2019, Carsten Sorensen and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <stdint.h>
+
+#include "asm/main.h"
+#include "asm/util.h"
+
+#include "extern/utf8decoder.h"
+
+/*
+ * Calculate the hash value for a string
+ */
+uint32_t calchash(const char *s)
+{
+	uint32_t hash = 5381;
+
+	while (*s != 0)
+		hash = (hash * 33) ^ (*s++);
+
+	return hash;
+}
+
+int32_t readUTF8Char(char *dest, char *src)
+{
+	uint32_t state;
+	uint32_t codep;
+	int32_t i;
+
+	for (i = 0, state = 0;; i++) {
+		if (decode(&state, &codep, (uint8_t)src[i]) == 1)
+			fatalerror("invalid UTF-8 character");
+
+		dest[i] = src[i];
+
+		if (state == 0) {
+			dest[++i] = '\0';
+			return i;
+		}
+	}
+}