Move parachain inherent data into its own crate (#326)

This renames and moves the `SystemInherentData` into its own crate.
The struct is now called `ParachainInherentData`. Besides moving the
struct, this also moves the code for creating this struct into this crate.

cumulus/primitives/parachain-inherent/src/lib.rs

@@ -0,0 +1,64 @@
+// Copyright 2021 Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate 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.
+
+// Substrate 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 Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Cumulus parachain inherent
+//!
+//! The [`ParachainInherentData`] is the data that is passed by the collator to the parachain runtime.
+//! The runtime will use this data to execute messages from other parachains/the relay chain or to
+//! read data from the relay chain state. When the parachain is validated by a parachain validator on
+//! the relay chain, this data is checked for correctnes. If the data passed by the collator to the
+//! runtime isn't correct, the parachain candidate is considered invalid.
+//!
+//! Use [`ParachainInherentData::create_at`] to create the [`ParachainInherentData`] at a given
+//! relay chain block to include it in a parachain block.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use cumulus_primitives_core::{
+	InboundDownwardMessage, InboundHrmpMessage, ParaId, PersistedValidationData,
+};
+
+use sp_inherents::InherentIdentifier;
+use sp_std::{collections::btree_map::BTreeMap, vec::Vec};
+
+#[cfg(feature = "std")]
+mod client_side;
+#[cfg(feature = "std")]
+pub use client_side::*;
+
+/// The identifier for the parachain inherent.
+pub const INHERENT_IDENTIFIER: InherentIdentifier = *b"sysi1337";
+
+/// The inherent data that is passed by the collator to the parachain runtime.
+#[derive(codec::Encode, codec::Decode, sp_core::RuntimeDebug, Clone, PartialEq)]
+pub struct ParachainInherentData {
+	pub validation_data: PersistedValidationData,
+	/// A storage proof of a predefined set of keys from the relay-chain.
+	///
+	/// Specifically this witness contains the data for:
+	///
+	/// - active host configuration as per the relay parent,
+	/// - the relay dispatch queue sizes
+	/// - the list of egress HRMP channels (in the list of recipients form)
+	/// - the metadata for the egress HRMP channels
+	pub relay_chain_state: sp_trie::StorageProof,
+	/// Downward messages in the order they were sent.
+	pub downward_messages: Vec<InboundDownwardMessage>,
+	/// HRMP messages grouped by channels. The messages in the inner vec must be in order they
+	/// were sent. In combination with the rule of no more than one message in a channel per block,
+	/// this means `sent_at` is **strictly** greater than the previous one (if any).
+	pub horizontal_messages: BTreeMap<ParaId, Vec<InboundHrmpMessage>>,
+}