Move ethereum-specific H256FastMap type to own crate (#9307)

* Add a `fastmap` crate that provides the H256FastMap specialized HashMap

* Use `fastmap` instead of `plain_hasher`

* Update submodules for Reasons™

* Submodule update

util/fastmap/src/lib.rs

@@ -0,0 +1,39 @@
+// Copyright 2015-2018 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity 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 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.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Provides a `H256FastMap` type with H256 keys and fast hashing function.
+
+extern crate ethereum_types;
+extern crate plain_hasher;
+
+use ethereum_types::H256;
+use std::hash;
+use std::collections::HashMap;
+use plain_hasher::PlainHasher;
+
+/// Specialized version of `HashMap` with H256 keys and fast hashing function.
+pub type H256FastMap<T> = HashMap<H256, T, hash::BuildHasherDefault<PlainHasher>>;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_works() {
+        let mut h = H256FastMap::default();
+        h.insert(H256::from(123), "abc");
+    }
+}