Add FungibleAdapter (#2684)

In the move from the old `Currency` traits to the new `fungible/s`
family of traits, we already had the `FungiblesAdapter` and
`NonFungiblesAdapter` for multiple fungible and non fungible assets
respectively. However, for handling only one fungible asset, we were
missing a `FungibleAdapter`, and so used the old `CurrencyAdapter`
instead. This PR aims to fill in that gap, and provide the new adapter
for more updated examples.

I marked the old `CurrencyAdapter` as deprecated as part of this PR, and
I'll change it to the new `FungibleAdapter` in a following PR.
The two stages are separated so as to not bloat this PR with some name
fixes in tests.

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Co-authored-by: command-bot <>

polkadot/xcm/pallet-xcm-benchmarks/src/fungible/mock.rs

@@ -103,6 +103,7 @@ impl xcm_executor::traits::MatchesFungible<u64> for MatchAnyFungible {
 }
 
 // Use balances as the asset transactor.
+#[allow(deprecated)]
 pub type AssetTransactor = xcm_builder::CurrencyAdapter<
 	Balances,
 	MatchAnyFungible,