implementing string indexes

samples/indices.lox

@@ -0,0 +1,3 @@
+var alphabet = "abcdefghijklmnopqrstuvwxyz"; var X = alphabet; print "hello " + X[3] + X[0] + X[10];
+
+var s = "   "; s[0] = "d"; s[1] = "a"; s[2] = "k"; var hey = "hello " + s + "!"; print hey;