Cleaner access to `:code` storage

[JoshOrndorff]

Currently the runtime upgrade piece has a side effect of writing directly into storage at :code.

It seems cleaner to me (at the moment, but this is up for discussion) to have an access trait for the runtime storage and make pieces like the runtime upgrade piece reference the write access through the necessary accessor method.

Something similar is how I handled the relay parent block number storage for the parachain runtime.

Tuxedo/tuxedo-parachain-core/src/lib.rs

Lines 93 to 97 in 1ad2343

	impl SetRelayParentNumberStorage for RelayParentNumberStorage {
	fn set(new_parent_number: u32) {
	sp_io::storage::set(RELAY_PARENT_NUMBER_KEY, &new_parent_number.encode());
	}
	}

Depending how many of these "special" storage keys there are, maybe we could even formalize a notion of global storage item. But I'm not ready to go there quite yet.