`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/4ec7496f/cumulus/parachains/runtimes/assets/asset-hub-kusama/src/lib.rs#L327),
[2](https://github.com/paritytech/polkadot-sdk/blob/4ec7496f

/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>