Update string_iterator.c3 to include extra convenience methods (#1327)

Update string_iterator.c3 to include extra convenience methods

Added peek: returns the next character without incrementing current
Added has_next: checks if the iterator has another element
Added get: gets the current element (the same one that was returned with the previous call to next).

test/unit/stdlib/core/string_iterator.c3

@@ -0,0 +1,35 @@
+module std::core::test::string_iterator::tests @test;
+
+fn void test_at_start()
+{
+    String test = "abcd";
+    StringIterator iterator = test.iterator();
+    assert(iterator.get()! == 'a');
+    assert(iterator.peek()! == 'a');
+    iterator.next()!;
+    assert(iterator.next()! == 'b');
+    assert(iterator.has_next());
+}
+
+
+fn void test_general()
+{ 
+    String test = "åƦs1";
+    StringIterator iterator = test.iterator();
+    assert(iterator.get()! == 'å');
+    iterator.next()!;
+    assert(iterator.peek()! == 'Ʀ');
+    assert(iterator.next()! == 'Ʀ');
+    iterator.reset();
+    assert(iterator.current == 0);
+}
+
+fn void test_end()
+{
+    String test = "åƦ";
+    StringIterator iterator = test.iterator();
+    assert(@ok(iterator.next()));
+    assert(iterator.peek()! == 'Ʀ');
+    assert(@ok(iterator.next()));
+    assert(@catch(iterator.next()));
+}