Adds support for checking the timestamp inherent while validating a block (#494)

* Adds support for checking the timestamp inherent while validating a block

This adds support for checking the timestamp inherent while validating a
block. This will use the relay chain slot number * relay chain slot
duration to calculate a timestamp. This timestamp is used to check the
timestamp in the timestamp inherent.

* Update polkadot-parachains/rococo-runtime/src/lib.rs

Co-authored-by: Shawn Tabrizi <[email protected]>

* Update polkadot-parachains/statemine-runtime/src/lib.rs

Co-authored-by: Shawn Tabrizi <[email protected]>

* Update primitives/timestamp/src/lib.rs

Co-authored-by: André Silva <[email protected]>

* Fix warnings

Co-authored-by: Shawn Tabrizi <[email protected]>
Co-authored-by: André Silva <[email protected]>