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

docs/integrations/Instructions.md

@@ -40,10 +40,10 @@ Serai token. If an Application Call, the encoded call will be executed.
 
 ### Refundable In Instruction
 
-  - `origin` (Option\<ExternalAddress>): Address, from the network of origin,
-which sent coins in.
-  - `instruction` (InInstruction):       The action to perform with the incoming
-coins.
+  - `origin`      (Option\<ExternalAddress>): Address, from the network of
+origin, which sent coins in.
+  - `instruction` (InInstruction):            The action to perform with the
+incoming coins.
 
 Networks may automatically provide `origin`. If they do, the instruction may
 still provide `origin`, overriding the automatically provided value.
@@ -51,19 +51,18 @@ still provide `origin`, overriding the automatically provided value.
 If the instruction fails, coins are scheduled to be returned to `origin`,
 if provided.
 
-### Destination
-
-Destination is an enum of SeraiAddress and ExternalAddress.
-
 ### Out Instruction
 
-  - `destination` (Destination):   Address to receive coins to.
-  - `data`        (Option\<Data>): The data to call the destination with.
+  - `address` (ExternalAddress): Address to transfer the coins included with
+this instruction to.
+  - `data`    (Option<Data>):    Data to include when transferring coins.
 
-Transfer the coins included with this instruction to the specified address with
-the specified data. No validation of external addresses/data is performed
-on-chain. If data is specified for a chain not supporting data, it is silently
-dropped.
+No validation of external addresses/data is performed on-chain. If data is
+specified for a chain not supporting data, it is silently dropped.
+
+### Destination
+
+Destination is an enum of SeraiAddress and OutInstruction.
 
 ### Shorthand
 
@@ -80,7 +79,7 @@ covered by Shorthand.
   - `origin`  (Option\<ExternalAddress>): Refundable In Instruction's `origin`.
   - `coin`    (Coin):                     Coin to swap funds for.
   - `minimum` (Amount):                   Minimum amount of `coin` to receive.
-  - `out`     (Out Instruction):          Final destination for funds.
+  - `out`     (Destination):              Final destination for funds.
 
 which expands to: