UnionOf types for merged fungible and fungibles implementations (#2033)

Introduces `UnionOf` types, crafted to merge `fungible` and `fungibles`
implementations or two `fungibles` implementations into a single type
implementing `fungibles`.

This also addresses an issue where `ItemOf` initiates a double drop for
an imbalance type, leading to inaccurate total issuance accounting.

Find the application of these types in this PR -
[link](https://github.com/paritytech/polkadot-sdk/pull/2031), places in
code -
[1](https://github.com/paritytech/polkadot-sdk/blob/4ec7496fa2632385b08fae860fcf28a523a7b5de/cumulus/parachains/runtimes/assets/asset-hub-kusama/src/lib.rs#L327),
[2](https://github.com/paritytech/polkadot-sdk/blob/4ec7496fa2632385b08fae860fcf28a523a7b5de/cumulus/parachains/runtimes/assets/asset-hub-kusama/src/lib.rs#L343).

---------

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: joepetrowski <joe@parity.io>

substrate/frame/assets/src/lib.rs

@@ -1648,8 +1648,20 @@ pub mod pallet {
 			T::AssetAccountDeposit::get()
 		}
 
-		fn touch(asset: T::AssetId, who: T::AccountId, depositor: T::AccountId) -> DispatchResult {
-			Self::do_touch(asset, who, depositor, false)
+		fn should_touch(asset: T::AssetId, who: &T::AccountId) -> bool {
+			match Asset::<T, I>::get(&asset) {
+				Some(info) if info.is_sufficient => false,
+				Some(_) => !Account::<T, I>::contains_key(asset, who),
+				_ => true,
+			}
+		}
+
+		fn touch(
+			asset: T::AssetId,
+			who: &T::AccountId,
+			depositor: &T::AccountId,
+		) -> DispatchResult {
+			Self::do_touch(asset, who.clone(), depositor.clone(), false)
 		}
 	}