Tokens pallet (#243)

* Use Monero-compatible additional TX keys

This still sends a fingerprinting flare up if you send to a subaddress which
needs to be fixed. Despite that, Monero no should no longer fail to scan TXs
from monero-serai regarding additional keys.

Previously it failed becuase we supplied one key as THE key, and n-1 as
additional. Monero expects n for additional.

This does correctly select when to use THE key versus when to use the additional
key when sending. That removes the ability for recipients to fingerprint
monero-serai by receiving to a standard address yet needing to use an additional
key.

* Add tokens_primitives

Moves OutInstruction from in-instructions.

Turns Destination into OutInstruction.

* Correct in-instructions DispatchClass

* Add initial tokens pallet

* Don't allow pallet addresses to equal identity

* Add support for InInstruction::transfer

Requires a cargo update due to modifications made to serai-dex/substrate.

Successfully mints a token to a SeraiAddress.

* Bind InInstructions to an amount

* Add a call filter to the runtime

Prevents worrying about calls to the assets pallet/generally tightens things
up.

* Restore Destination

It was meged into OutInstruction, yet it didn't make sense for OutInstruction
to contain a SeraiAddress.

Also deletes the excessively dated Scenarios doc.

* Split PublicKey/SeraiAddress

Lets us define a custom Display/ToString for SeraiAddress.

Also resolves an oddity where PublicKey would be encoded as String, not
[u8; 32].

* Test burning tokens/retrieving OutInstructions

Modularizes processor_coinUpdates into a shared testing utility.

* Misc lint

* Don't use PolkadotExtrinsicParams

substrate/node/src/rpc.rs

@@ -6,7 +6,11 @@ use sp_blockchain::{Error as BlockchainError, HeaderBackend, HeaderMetadata};
 use sp_block_builder::BlockBuilder;
 use sp_api::ProvideRuntimeApi;
 
-use serai_runtime::{primitives::SeraiAddress, opaque::Block, Balance, Index};
+use serai_runtime::{
+  primitives::{SubstrateAmount, PublicKey},
+  opaque::Block,
+  Index,
+};
 
 pub use sc_rpc_api::DenyUnsafe;
 use sc_transaction_pool_api::TransactionPool;
@@ -29,8 +33,8 @@ pub fn create_full<
   deps: FullDeps<C, P>,
 ) -> Result<RpcModule<()>, Box<dyn std::error::Error + Send + Sync>>
 where
-  C::Api: substrate_frame_rpc_system::AccountNonceApi<Block, SeraiAddress, Index>
-    + pallet_transaction_payment_rpc::TransactionPaymentRuntimeApi<Block, Balance>
+  C::Api: substrate_frame_rpc_system::AccountNonceApi<Block, PublicKey, Index>
+    + pallet_transaction_payment_rpc::TransactionPaymentRuntimeApi<Block, SubstrateAmount>
     + BlockBuilder<Block>,
 {
   use substrate_frame_rpc_system::{System, SystemApiServer};