Use Treadle (on pull requests only) to speed up CI

Treadle will be used as the "defaultBackend" when the environment
variable CHISEL3_CI_USE_TREADLE is set. The intent is to set this
variable during CI for pre-merge CI (aka on pull requests).

The command line options for CIRCT's firtool command have been updated.

Previously, failed jobs in the CI matrix would cause the sentinel job
(all-tests-passed) to be skipped, which for purposes of Github Actions
branch protection would count as "success". This allowed PRs with
failing CI to be merged. This new approach which uses two sentinel jobs
should not suffer from this same issue.

This enables emission of modules that serialize to >2 GiB of .fir text.

* Cleanup Cookbook and printing docs
* Format specifiers are actually concise now

Co-authored-by: Megan Wachs <megan@sifive.com>

Referenced to: chipsalliance/chisel3#2728

Add macro for determining Chisel structural subtypes. This replaces the                                                                                                                                                       
use of built-in <:< for determining what types can be cast with
.viewAsSuperType.

Co-authored-by: Jack Koenig <koenig@sifive.com>

Mem.read is combinational and thus unaffected by the clock, and so it
does not make sense to issue warnings about the current clock in this
context.

This reverts commit de76e70b.

- Remove line defeating having wire locators
  `implicit val noSourceInfo = UnlocatableSourceInfo` from `WireDefault#apply`
- Add test to show locators

We need to ensure that contextual names stay contextual (ie. sensitive
to the module context which is important for naming ports).

Also make the specified version more human readable for easier bumping
in the future.

Co-authored-by: Jack Koenig <jack.koenig3@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Adam Izraelevitz <adam.izraelevitz@sifive.com>

* Add a cookbook and publicly visible scaladoc for prefix, noPrefix

Fix Record cloneTypes.

Add support for printable within assert and assume verification statements

Co-authored-by: Girish Pai <girish.pai@sifive.com>
Co-authored-by: Megan Wachs <megan@sifive.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>

OpaqueTypes are essentially type aliases that hide the underlying type.
They are implemented in Chisel as Records of a single, unnamed element
where the wrapping Record does not exist in the emitted FIRRTL.

Co-authored-by: Jack Koenig <koenig@sifive.com>

Add --warnings-as-errors option

Co-authored-by: Jack Koenig <koenig@sifive.com>

Upstream sifive/chisel-circt.

Or: now we're cooking with MLIR.

Use the published Ubuntu 20.04 binary of CIRCT to enable chisel-circt
tests in CI.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>