* feat[contracts]: add GasPriceOracle w/o predeploy

Based on #912

* feat[contracts]: congestion price oracle

* chore: add changeset

* contracts: gas price oracle (#917)

* contracts: gas price oracle

* tests: update

* fees: fix tests

* contracts: simplify gas price oracle

* lint: fix

* test: execution price is at the 1st storage slot

* chore: rename predeploy to GasPriceOracle

* chore: rename gas price oracle test name
Co-authored-by: Mark Tyneway <mark.tyneway@gmail.com>
Co-authored-by: Georgios Konstantopoulos <me@gakonst.com>

* Add an L2 deploy script for gas oracle contract

* Add a kovan deployment artifact

* Add deployment to readme

* Add extra validation & initial execution price

* Update README.md

* Fix execution price logic

* Perform new deployment with final contract

* contracts: better require in ovm gas price oracle

* Deploy L2GasPriceOracle

* Update contract to use new fee logic & rename to gas

* Deploy updated contract

* Fix lint

* gas price oracle: do not restrict gas price

* gas price oracle: new deployment

* tests: delete dead test
Co-authored-by: smartcontracts <kelvinfichter@gmail.com>
Co-authored-by: Mark Tyneway <mark.tyneway@gmail.com>
Co-authored-by: Georgios Konstantopoulos <me@gakonst.com>