[NPoS] Fix for Reward Deficit in the pool (#1255)

closes https://github.com/paritytech/polkadot-sdk/issues/158.
partially addresses
https://github.com/paritytech/polkadot-sdk/issues/226.

Instead of fragile calculation of current balance by looking at `free
balance - ED`, Nomination Pool now freezes ED in the pool reward account
to restrict an account from going below minimum balance. This also has a
nice side effect that if ED changes, we know how much is the imbalance
in ED frozen in the pool and the current required ED. A pool operator
can diligently top up the pool with the deficit in ED or vice versa,
withdraw the excess they transferred to the pool.

## Notable changes
- New call `adjust_pool_deposit`: Allows to top up the deficit or
withdraw the excess deposited funds to the pool.
- Uses Fungible trait (instead of Currency trait). Since NP was not
doing any locking/reserving previously, no migration is needed for this.
- One time migration of freezing ED from each of the existing pools (not
very PoV friendly but fine for relay chain).

substrate/frame/support/src/traits/tokens/misc.rs

@@ -20,7 +20,10 @@
 use codec::{Decode, Encode, FullCodec, MaxEncodedLen};
 use sp_arithmetic::traits::{AtLeast32BitUnsigned, Zero};
 use sp_core::RuntimeDebug;
-use sp_runtime::{traits::Convert, ArithmeticError, DispatchError, TokenError};
+use sp_runtime::{
+	traits::{Convert, MaybeSerializeDeserialize},
+	ArithmeticError, DispatchError, TokenError,
+};
 use sp_std::fmt::Debug;
 
 /// The origin of funds to be used for a deposit operation.
@@ -240,6 +243,7 @@ pub trait Balance:
 	+ MaxEncodedLen
 	+ Send
 	+ Sync
+	+ MaybeSerializeDeserialize
 	+ 'static
 {
 }
@@ -253,6 +257,7 @@ impl<
 			+ MaxEncodedLen
 			+ Send
 			+ Sync
+			+ MaybeSerializeDeserialize
 			+ 'static,
 	> Balance for T
 {