Make strings iterable over their code points.

I'm not sure why, but this also regresses perf:

binary_trees - wren            ..........  3290  0.30s   96.68% relative to baseline
delta_blue - wren              ..........  7948  0.13s   99.06% relative to baseline
fib - wren                     ..........  3165  0.32s   95.90% relative to baseline
for - wren                     ..........  8242  0.12s   96.00% relative to baseline
method_call - wren             ..........  5417  0.18s   78.74% relative to baseline

Need to investigate.

test/list/iterate.wren

@@ -4,6 +4,9 @@ IO.print(a.iterate(0)) // expect: 1
 IO.print(a.iterate(1)) // expect: 2
 IO.print(a.iterate(2)) // expect: 3
 IO.print(a.iterate(3)) // expect: false
+
+// Out of bounds.
+IO.print(a.iterate(123)) // expect: false
 IO.print(a.iterate(-1)) // expect: false
 
 // Nothing to iterate in an empty list.