Add child tries support in host.rs (#167)

* Add child tries support in host.rs

* Fix borrowck issues in upper layers

* Fix borrow checker issues in host.rs

* Finish ExternalStorageRoot and ext_storage_exists_version_1

* Handle the commit changes trick

* Implement ext_default_child_storage_get_version_1, ext_default_child_storage_read_version_1, ext_default_child_storage_exists_version_1

* Implement ext_default_child_storage_root_version_1 and ext_default_child_storage_root_version_2

* Implement ext_default_child_storage_set_version_1 and ext_default_child_storage_clear_version_1

* Implement ext_default_child_storage_next_key_version_1

* Implement ext_default_child_storage_clear_prefix_version_1 and ext_default_child_storage_clear_prefix_version_2

* Implement ext_default_child_storage_storage_kill

* Clean up ExternalStorageClearPrefix

* Add parameters count

* Add custom Debug impl for StorageKey