Add string operations

tests/string.grnd

@@ -0,0 +1,11 @@
+input &str
+getstrsize $str &size
+set &idx 0
+@loop
+getstrcharat $str $idx &char
+println $char
+add 1 $idx &idx
+equal $idx $size &cond
+if $cond %loopend
+jump %loop
+@loopend