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/tokens/pallet/Cargo.toml

@@ -0,0 +1,38 @@
+[package]
+name = "tokens-pallet"
+version = "0.1.0"
+description = "Mint and burn Serai tokens"
+license = "AGPL-3.0-only"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+edition = "2021"
+publish = false
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
+[dependencies]
+scale = { package = "parity-scale-codec", version = "3", default-features = false, features = ["derive", "max-encoded-len"] }
+scale-info = { version = "2", default-features = false, features = ["derive"] }
+
+frame-system = { git = "https://github.com/serai-dex/substrate", default-features = false }
+frame-support = { git = "https://github.com/serai-dex/substrate", default-features = false }
+
+pallet-assets = { git = "https://github.com/serai-dex/substrate", default-features = false }
+
+serai-primitives = { path = "../../serai/primitives", default-features = false }
+tokens-primitives = { path = "../primitives", default-features = false }
+
+[features]
+std = [
+  "scale/std",
+  "scale-info/std",
+
+  "frame-system/std",
+  "frame-support/std",
+
+  "pallet-assets/std",
+
+  "serai-primitives/std",
+]
+default = ["std"]