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

ethcore/sync/src/lib.rs

@@ -31,7 +31,7 @@ extern crate ethcore;
 extern crate ethereum_types;
 extern crate env_logger;
 extern crate hashdb;
-extern crate plain_hasher;
+extern crate fastmap;
 extern crate rand;
 extern crate semver;
 extern crate parking_lot;