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 <shawntabrizi@gmail.com>

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

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update primitives/timestamp/src/lib.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Fix warnings

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>