Implement new string functions (#1655)

`STRFIND`, `STRRFIND`, `STRCHAR`, `STRSLICE`, `CHARCMP`, `CHARSIZE`, and `REVCHAR`

test/asm/charlen-strchar.asm

@@ -0,0 +1,34 @@
+	opt Wno-unmapped-char
+	charmap "<NULL>", $00
+	charmap "A", $10
+	charmap "B", $20
+	charmap "C", $30
+	charmap "Bold", $88
+
+SECTION "test", ROM0
+
+DEF S EQUS "XBold<NULL>ABC"
+
+	assert CHARLEN("{S}") == 6
+	println STRCHAR("{S}", 1)
+
+	assert !STRCMP(STRCHAR("{S}", 1), "Bold")
+	assert STRCHAR("{S}", -5) == STRCHAR("{S}", CHARLEN("{S}") - 5)
+	assert STRCHAR("{S}", 1) == "Bold" && "Bold" == $88
+	assert STRCHAR("{S}", 0) == $58 ; ASCII "X"
+	db "{S}"
+
+	for n, CHARLEN("{S}")
+		assert STRCHAR("{S}", n) == CHARSUB("{S}", n + 1)
+		assert STRCHAR("{S}", -n - 1) == CHARSUB("{S}", -n - 1)
+	endr
+
+	newcharmap ascii
+
+	assert CHARLEN("{S}") == 14
+	println STRCHAR("{S}", 1)
+	assert !STRCMP(STRCHAR("{S}", 1), "B")
+	assert STRCHAR("{S}", -5) == STRCHAR("{S}", CHARLEN("{S}") - 5)
+	assert STRCHAR("{S}", 1) == "B" && "B" == $42 ; ASCII "B"
+	assert STRCHAR("{S}", 0) == $58 ; ASCII "X"
+	db "{S}"