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.

test/asm/multiple-charmaps.asm

@@ -0,0 +1,88 @@
+printt "main charmap\n"
+
+charmap "ab", $0
+
+x = "ab"
+printt "{x}\n"
+
+printt "newcharmap map1\n"
+newcharmap map1
+
+x = "ab"
+printt "{x}\n"
+
+printt "newcharmap map2, main\n"
+newcharmap map2, main
+
+x = "ab"
+printt "{x}\n"
+
+printt "setcharmap map1\n"
+setcharmap map1
+
+x = "ab"
+printt "{x}\n"
+
+printt "newcharmap map3\n"
+newcharmap map3
+
+charmap "ab", $1
+
+x = "ab"
+printt "{x}\n"
+
+printt "newcharmap map4, map3\n"
+newcharmap map4, map3
+
+charmap "ab", $1
+charmap "cd", $2
+
+x = "ab"
+printt "{x}\n"
+
+x = "cd"
+printt "{x}\n"
+
+printt "setcharmap map3\n"
+setcharmap map3
+
+x = "ab"
+printt "{x}\n"
+
+x = "cd"
+printt "{x}\n"
+
+printt "setcharmap main\n"
+setcharmap main
+
+SECTION "sec0", ROM0
+
+x = "ab"
+printt "{x}\n"
+
+printt "override main charmap\n"
+charmap "ef", $3
+
+x = "ab"
+printt "{x}\n"
+
+x = "ef"
+printt "{x}\n"
+
+printt "setcharmap map3\n"
+setcharmap map3
+
+x = "ab"
+printt "{x}\n"
+
+x = "cd"
+printt "{x}\n"
+
+x = "ef"
+printt "{x}\n"
+
+printt "newcharmap map1\n"
+newcharmap map1
+
+printt "setcharmap map5\n"
+setcharmap map5