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/equ-charmap.asm

@@ -1,4 +1,4 @@
-SECTION "sec", ROM0
 	charmap "A", 1
+SECTION "sec", ROM0
 _A_ EQU "A"
 	db _A_

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

test/asm/multiple-charmaps.out

@@ -0,0 +1,34 @@
+warning: multiple-charmaps.asm(64):
+    Using 'charmap' within a section when the current charmap is 'main' is deprecated
+ERROR: multiple-charmaps.asm(85):
+    Charmap 'map1' already exists
+ERROR: multiple-charmaps.asm(88):
+    Charmap 'map5' doesn't exist
+error: Assembly aborted (2 errors)!
+main charmap
+$0
+newcharmap map1
+$6162
+newcharmap map2, main
+$0
+setcharmap map1
+$6162
+newcharmap map3
+$1
+newcharmap map4, map3
+$1
+$2
+setcharmap map3
+$1
+$6364
+setcharmap main
+$0
+override main charmap
+$6162
+$3
+setcharmap map3
+$1
+$6364
+$6566
+newcharmap map1
+setcharmap map5

test/asm/multiple-charmaps.out.pipe

@@ -0,0 +1,34 @@
+warning: -(64):
+    Using 'charmap' within a section when the current charmap is 'main' is deprecated
+ERROR: -(85):
+    Charmap 'map1' already exists
+ERROR: -(88):
+    Charmap 'map5' doesn't exist
+error: Assembly aborted (2 errors)!
+main charmap
+$0
+newcharmap map1
+$6162
+newcharmap map2, main
+$0
+setcharmap map1
+$6162
+newcharmap map3
+$1
+newcharmap map4, map3
+$1
+$2
+setcharmap map3
+$1
+$6364
+setcharmap main
+$0
+override main charmap
+$6162
+$3
+setcharmap map3
+$1
+$6364
+$6566
+newcharmap map1
+setcharmap map5