Implement new string functions (#1655)

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

test/asm/charcmp.asm

@@ -0,0 +1,27 @@
+charmap "a", 1
+charmap "b", 2
+charmap "c", 0
+charmap "w", 3, 2, 1
+charmap "x", 1, 2
+charmap "y", 2, 1
+charmap "z", 1, 2, 3
+
+macro test
+	println strfmt("\"%#s\" <=> \"%#s\" == %d", \1, \2, charcmp(\1, \2))
+endm
+
+test "", ""
+test "a", "a"
+test "aa", "aaa"
+test "aaa", "aa"
+test "a", "b"
+test "b", "a"
+test "", "b"
+test "c", ""
+test "abc", "cba"
+test "cabc", "cxc"
+test "zy", "abw"
+test "abab", "xx"
+test "abab", "ww"
+test "w", "z"
+test "xcy", "zw"