Implement CHARLEN and CHARSUB

Fixes #786

test/asm/charlen-charsub.asm

@@ -0,0 +1,25 @@
+	charmap "<NULL>", $00
+	charmap "A", $10
+	charmap "B", $20
+	charmap "C", $30
+	charmap "Bold", $88
+
+SECTION "test", ROM0
+
+S EQUS "XBold<NULL>ABC"
+
+	assert CHARLEN("{S}") == 6
+	println CHARSUB("{S}", 2)
+	assert !STRCMP(CHARSUB("{S}", 2), "Bold")
+	assert CHARSUB("{S}", 2) == "Bold" && "Bold" == $88
+	assert CHARSUB("{S}", 1) == $58 ; ASCII "X"
+	db "{S}"
+
+	newcharmap ascii
+
+	assert CHARLEN("{S}") == 14
+	println CHARSUB("{S}", 2)
+	assert !STRCMP(CHARSUB("{S}", 2), "B")
+	assert CHARSUB("{S}", 2) == "B" && "B" == $42 ; ASCII "B"
+	assert CHARSUB("{S}", 1) == $58 ; ASCII "X"
+	db "{S}"

test/asm/charlen-charsub.out

@@ -0,0 +1,2 @@
+Bold
+B