Coins pallet (#399)

* initial implementation

* add function to get a balance of an account

* add support for multiple coins

* rename pallet to "coins-pallet"

* replace balances, assets and tokens pallet with coins pallet in runtime

* add total supply info

* update client side for new Coins pallet

* handle fees

* bug fixes

* Update FeeAccount test

* Fmt

* fix pr comments

* remove extraneous Imbalance type

* Minor tweaks

---------

Co-authored-by: Luke Parker <lukeparker5132@gmail.com>

substrate/coins/primitives/src/lib.rs

@@ -0,0 +1,51 @@
+#![cfg_attr(docsrs, feature(doc_cfg))]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(feature = "std")]
+use zeroize::Zeroize;
+
+use serde::{Serialize, Deserialize};
+
+use scale::{Encode, Decode, MaxEncodedLen};
+use scale_info::TypeInfo;
+
+use serai_primitives::{Balance, SeraiAddress, ExternalAddress, Data, system_address};
+
+pub const FEE_ACCOUNT: SeraiAddress = system_address(b"FeeAccount");
+
+#[derive(
+  Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Encode, Decode, MaxEncodedLen, TypeInfo,
+)]
+#[cfg_attr(feature = "std", derive(Zeroize))]
+pub struct OutInstruction {
+  pub address: ExternalAddress,
+  pub data: Option<Data>,
+}
+
+#[derive(
+  Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Encode, Decode, MaxEncodedLen, TypeInfo,
+)]
+#[cfg_attr(feature = "std", derive(Zeroize))]
+pub struct OutInstructionWithBalance {
+  pub instruction: OutInstruction,
+  pub balance: Balance,
+}
+
+#[derive(
+  Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Encode, Decode, MaxEncodedLen, TypeInfo,
+)]
+#[cfg_attr(feature = "std", derive(Zeroize))]
+pub enum Destination {
+  Native(SeraiAddress),
+  External(OutInstruction),
+}
+
+#[test]
+fn address() {
+  use sp_runtime::traits::TrailingZeroInput;
+  assert_eq!(
+    FEE_ACCOUNT,
+    SeraiAddress::decode(&mut TrailingZeroInput::new(b"FeeAccount")).unwrap()
+  );
+}