Invalidateapi (#645)

* Require explicit connection to DontCare to generate "is invalid".

* Add tests for RefNotInitializedException.
Currently, we fail the when ... otherwise ...

* Disable ScalaTest shrinking on error in ComplexAssignSpec.

* fix broken merge; still some binding issues

* cleanup DontCare connection checks; add missing directions to test module IOs

* Have library code inherit compileOptions from the enclosing Module (if it exists).

* work around current firrtl uninitialized references with Strict compile options and explicitInvalidate

* more CompileOptions cleanup; move test-specific defines to package object

* minimize differences with master

* set default CompileOptions.explicitInvalidate to false until we fix the FIRRTL when issue

* ignore the StrictCompiler property checks (until  CompileOptions.explicitInvalidate is defaulted to true)

* Revert "more CompileOptions cleanup; move test-specific defines to package object"

This reverts commit e4486edcba990d150e76e08a2fc6abca033556e0.

* Revert "work around current firrtl uninitialized references with Strict compile options and explicitInvalidate"

This reverts commit 426faa430a62c3dac2dbdf33044d3386d4243157.

* remove unused code

* Convert to binding-based DontCare implementation

* comment cleanup to minimize differences with master

* Tentatively remove possibly redundant DefInvalid on module ports.

* Respond to code review change request.
- backout build.sbt change
- correct indentation
- handle bulk of DontCare semantics in elemConnect()
- have DontCare extend Element, not Data (eliminate most Object specific methods
- add comments indicating reason for explicit DontCare connections

* Initialize test elements without requiring a DontCare.

* Respond to review change requests.
- DontCare should work on left or right side in BiDirectional connections
- call bind() to set DontCare binding instead of messing with internal variables
- DontCares are only equivalent with DontCares
- clean up processWhens() definition

* Eliminate DontCare connection to inputs in MonoConnect().

* Pull aggregates apart for the purpose of DontCare connections.

* Restore the explicit (conditionally executed) ports DefInvalidin ImplicitModule()

* Don't add DontCare's to the module list of _ids.

* Add missing DefInvalid() to LegacyModule().

* Respond to review requests: add DontCare BiConnect Vec, remove null parent hack to avoid addId(), initialize singletons early in Builder

* Move DontCare out of chisel3.experimental.