Generic keystore (#3008)

* Add KeyTypeId.

* Implement clone for sr25519::Pair.

* Extend Pair with to_raw_vec.

* Implement TypedKey for Signature and Pair.

* Add trait Public.

* Make keystore generic.

* Fixup clone.

* Fix tests.

* Update service.

* Fix imports.

* Fix build.

* Fix babe build.

* Fix subkey build.

* Make authority setup generic.

* Update node-template.

* Fix build.

* Remove unsafe code.

* Fix tests.