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.

test/string/iterate.wren

@@ -0,0 +1,16 @@
+var s = "abçd"
+IO.print(s.iterate(null)) // expect: 0
+IO.print(s.iterate(0)) // expect: 1
+IO.print(s.iterate(1)) // expect: 2
+// Skip 3 because that's the middle of the ç sequence.
+IO.print(s.iterate(2)) // expect: 4
+// Iterating from the middle of a UTF-8 sequence goes to the next one.
+IO.print(s.iterate(3)) // expect: 4
+IO.print(s.iterate(4)) // expect: false
+
+// Out of bounds.
+IO.print(s.iterate(123)) // expect: false
+IO.print(s.iterate(-1)) // expect: false
+
+// Nothing to iterate in an empty string.
+IO.print("".iterate(null)) // expect: false

test/string/iterate_iterator_not_int.wren

@@ -0,0 +1 @@
+"s".iterate(1.5) // expect runtime error: Iterator must be an integer.

test/string/iterate_iterator_not_num.wren

@@ -0,0 +1 @@
+"s".iterate("2") // expect runtime error: Iterator must be a number.

test/string/iterator_value.wren

@@ -0,0 +1,7 @@
+var s = "abçd"
+IO.print(s.iteratorValue(0)) // expect: a
+IO.print(s.iteratorValue(1)) // expect: b
+IO.print(s.iteratorValue(2)) // expect: ç
+// Iterator value in middle of UTF sequence is an empty string.
+IO.print(s.iteratorValue(3) == "") // expect: true
+IO.print(s.iteratorValue(4)) // expect: d

test/string/iterator_value_iterator_not_int.wren

@@ -0,0 +1 @@
+"s".iteratorValue(1.5) // expect runtime error: Iterator must be an integer.

test/string/iterator_value_iterator_not_num.wren

@@ -0,0 +1 @@
+"s".iteratorValue("2") // expect runtime error: Iterator must be a number.

test/string/iterator_value_iterator_too_large.wren

@@ -0,0 +1 @@
+"123".iteratorValue(4) // expect runtime error: Iterator out of bounds.

test/string/iterator_value_iterator_too_small.wren

@@ -0,0 +1 @@
+"123".iteratorValue(-5) // expect runtime error: Iterator out of bounds.