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.

---------

Co-authored-by: command-bot <>

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

@@ -32,13 +32,15 @@ pub use sp_std::{
 	cell::RefCell, collections::btree_map::BTreeMap, fmt::Debug, marker::PhantomData,
 };
 use xcm::prelude::*;
+#[allow(deprecated)]
+use xcm_builder::CurrencyAdapter as XcmCurrencyAdapter;
 use xcm_builder::{
 	AccountId32Aliases, AllowKnownQueryResponses, AllowSubscriptionsFrom,
 	AllowTopLevelPaidExecutionFrom, Case, ChildParachainAsNative, ChildParachainConvertsVia,
-	ChildSystemParachainAsSuperuser, CurrencyAdapter as XcmCurrencyAdapter, DescribeAllTerminal,
-	FixedRateOfFungible, FixedWeightBounds, FungiblesAdapter, HashedDescription, IsConcrete,
-	MatchedConvertedConcreteId, NoChecking, SignedAccountId32AsNative, SignedToAccountId32,
-	SovereignSignedViaLocation, TakeWeightCredit, XcmFeeManagerFromComponents, XcmFeeToAccount,
+	ChildSystemParachainAsSuperuser, DescribeAllTerminal, FixedRateOfFungible, FixedWeightBounds,
+	FungiblesAdapter, HashedDescription, IsConcrete, MatchedConvertedConcreteId, NoChecking,
+	SignedAccountId32AsNative, SignedToAccountId32, SovereignSignedViaLocation, TakeWeightCredit,
+	XcmFeeManagerFromComponents, XcmFeeToAccount,
 };
 use xcm_executor::{
 	traits::{Identity, JustTry},
@@ -428,6 +430,7 @@ pub type ForeignAssetsConvertedConcreteId = MatchedConvertedConcreteId<
 	JustTry,
 >;
 
+#[allow(deprecated)]
 pub type AssetTransactors = (
 	XcmCurrencyAdapter<Balances, IsConcrete<RelayLocation>, SovereignAccountOf, AccountId, ()>,
 	FungiblesAdapter<