Basic model checking API (#1499)

* Add `check(...)` affordance

* Add assert (renamed from check and fixed)

* Add verification statements

* Move formal to experimental.verification

* Make test use ChiselStage

`generateFirrtl` has been cut from Chisel

* Fix newly introduced style warnings

* Fix some old style warnings for good measure

* Revert "Fix some old style warnings for good measure"

This reverts commit 31d51726c2faa4c277230104bd469ff7ffefc890.

* Cut scalastyle comments

* Cut formal delimiter comments