Move std::array_iter to std::array::extensions

This way we use the same pattern as we do for extending Integer.

runtime/src/core/prelude.inko

@@ -27,14 +27,14 @@ import std::nil
 import std::block
 import std::string
 import std::array
-import std::array_iter
+import std::array::extensions::(self as _)
 import std::map::(Map as _Map)
 import std::inspect
 import std::byte_array
 import std::process
 import std::vm
 import std::range::(Range as _Range)
-import std::integer::extensions
+import std::integer::extensions::(self as _)
 import std::module
 # These constants are re-exported so they're available to all modules by

runtime/src/std/array_iter.inko → runtime/src/std/array/extensions.inko

-# Iterator types and methods for the `Array` type.
+# Extensions for the `Array` type that can only be defined later on in the
+# bootstrapping process.
 #
 # These types and methods are defined separately as otherwise `std::iterator`
 # would depend on `std::array` while `std::array` would depend on