Adding Bridges code as git subtree. (#2515)

* Add instructions.

* Squashed 'bridges/' content from commit 345e84a21

git-subtree-dir: bridges
git-subtree-split: 345e84a2146b56628e9888c9f5e129cb40e868a9

* Remove bridges workspace file to avoid confusing Cargo.

* Add some bridges primitives to Polkadot workspace.

* Improve docs.

polkadot/bridges/primitives/runtime/src/chain.rs

@@ -0,0 +1,87 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use frame_support::Parameter;
+use num_traits::AsPrimitive;
+use sp_runtime::traits::{
+	AtLeast32BitUnsigned, Hash as HashT, Header as HeaderT, MaybeDisplay, MaybeMallocSizeOf, MaybeSerializeDeserialize,
+	Member, SimpleBitOps,
+};
+use sp_std::str::FromStr;
+
+/// Minimal Substrate-based chain representation that may be used from no_std environment.
+pub trait Chain: Send + Sync + 'static {
+	/// A type that fulfills the abstract idea of what a Substrate block number is.
+	// Constraits come from the associated Number type of `sp_runtime::traits::Header`
+	// See here for more info:
+	// https://crates.parity.io/sp_runtime/traits/trait.Header.html#associatedtype.Number
+	//
+	// Note that the `AsPrimitive<usize>` trait is required by the GRANDPA justification
+	// verifier, and is not usually part of a Substrate Header's Number type.
+	type BlockNumber: Parameter
+		+ Member
+		+ MaybeSerializeDeserialize
+		+ sp_std::hash::Hash
+		+ Copy
+		+ Default
+		+ MaybeDisplay
+		+ AtLeast32BitUnsigned
+		+ FromStr
+		+ MaybeMallocSizeOf
+		+ AsPrimitive<usize>
+		+ Default;
+
+	/// A type that fulfills the abstract idea of what a Substrate hash is.
+	// Constraits come from the associated Hash type of `sp_runtime::traits::Header`
+	// See here for more info:
+	// https://crates.parity.io/sp_runtime/traits/trait.Header.html#associatedtype.Hash
+	type Hash: Parameter
+		+ Member
+		+ MaybeSerializeDeserialize
+		+ sp_std::hash::Hash
+		+ Ord
+		+ Copy
+		+ MaybeDisplay
+		+ Default
+		+ SimpleBitOps
+		+ AsRef<[u8]>
+		+ AsMut<[u8]>
+		+ MaybeMallocSizeOf;
+
+	/// A type that fulfills the abstract idea of what a Substrate hasher (a type
+	/// that produces hashes) is.
+	// Constraits come from the associated Hashing type of `sp_runtime::traits::Header`
+	// See here for more info:
+	// https://crates.parity.io/sp_runtime/traits/trait.Header.html#associatedtype.Hashing
+	type Hasher: HashT<Output = Self::Hash>;
+
+	/// A type that fulfills the abstract idea of what a Substrate header is.
+	// See here for more info:
+	// https://crates.parity.io/sp_runtime/traits/trait.Header.html
+	type Header: Parameter + HeaderT<Number = Self::BlockNumber, Hash = Self::Hash> + MaybeSerializeDeserialize;
+}
+
+/// Block number used by the chain.
+pub type BlockNumberOf<C> = <C as Chain>::BlockNumber;
+
+/// Hash type used by the chain.
+pub type HashOf<C> = <C as Chain>::Hash;
+
+/// Hasher type used by the chain.
+pub type HasherOf<C> = <C as Chain>::Hasher;
+
+/// Header type used by the chain.
+pub type HeaderOf<C> = <C as Chain>::Header;