Implement INCHARMAP function (#1184)

test/asm/incharmap.asm

@@ -0,0 +1,28 @@
+; default charmap 'main'
+charmap "a", 1
+charmap "ab", 2
+charmap "abc", 3
+
+newcharmap second
+charmap "d", 4
+charmap "e", 5
+charmap {__ISO_8601_UTC__}, 6 ; expands with quotes
+
+setcharmap main
+
+assert incharmap("a")
+assert incharmap("ab")
+assert incharmap(strcat("ab", "c"))
+
+assert !incharmap("") ; empty
+assert !incharmap("A") ; case sensitive
+assert !incharmap("aa") ; multiple chars "a" "a"
+assert !incharmap("d") ; unmapped char "d"
+assert !incharmap("bc") ; unmapped chars "b" "c"
+
+setcharmap second
+
+assert incharmap("d") ; now "d" is mapped
+assert !incharmap("a") ; only in 'main'
+assert !incharmap("bc") ; still unmapped chars
+assert incharmap({__ISO_8601_UTC__})