Fix folder names in primitives (#4358)

* sr-arithmetic -> arithmetic

* sr-sandbox -> sandbox

* primitives/sr-staking-primitives -> primitives/staking

* primitives/sr-version -> primitives/version

* primitives/block-builder/runtime-api -> primitives/block-builder

substrate/primitives/staking/src/lib.rs

@@ -0,0 +1,23 @@
+
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// 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.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+//! A crate which contains primitives that are useful for implementation that uses staking
+//! approaches in general. Definitions related to sessions, slashing, etc go here.
+
+pub mod offence;
+
+/// Simple index type with which we can count sessions.
+pub type SessionIndex = u32;