Implemented the following attributes:

* String.length
* String.asUpper
* String.asLower
* array.length
* array.pushBack(x)
* array.popBack()

The remaining attributes are listed in 'toy_attributes.h'

source/toy_attributes.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "toy_value.h"
+#include "toy_vm.h"
+
+Toy_Value handleStringAttributes(Toy_VM* vm, Toy_Value compound, Toy_Value attribute);
+Toy_Value handleArrayAttributes(Toy_VM* vm, Toy_Value compound, Toy_Value attribute);
+Toy_Value handleTableAttributes(Toy_VM* vm, Toy_Value compound, Toy_Value attribute);
+
+// string.length
+// string.asUpper
+// string.asLower
+// array.length
+// array.pushBack(x)
+// array.popBack()
+// array.forEach(fn) // fn(x) -> void
+// array.sort(fn)    // fn(a,b) -> int
+// table.length
+// table.insert(x)
+// table.hasKey(x)
+// table.remove(x)
+// table.forEach(fn) // fn(x) -> void