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/xcm-builder/src/lib.rs

@@ -65,6 +65,7 @@ mod process_xcm_message;
 pub use process_xcm_message::ProcessXcmMessage;
 
 mod currency_adapter;
+#[allow(deprecated)]
 pub use currency_adapter::CurrencyAdapter;
 
 mod fee_handling;
@@ -72,6 +73,9 @@ pub use fee_handling::{
 	deposit_or_burn_fee, HandleFee, XcmFeeManagerFromComponents, XcmFeeToAccount,
 };
 
+mod fungible_adapter;
+pub use fungible_adapter::{FungibleAdapter, FungibleMutateAdapter, FungibleTransferAdapter};
+
 mod fungibles_adapter;
 pub use fungibles_adapter::{
 	AssetChecking, DualMint, FungiblesAdapter, FungiblesMutateAdapter, FungiblesTransferAdapter,