Prevents infinite recursion on recursive types

Publish plugin for Scala versions 2.12.2-2.12.11
Also move plugin code to src/main/scala-2.12 so that the plugin is empty
when building for Scala 2.11

Remove Deprecated Usages of chisel3.Driver, CircuitForm

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

Co-authored-by: Chick Markley <chick@qrhino.com>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

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

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

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

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

This migrates the tests to Chisel 3.4/FIRRTL 1.4. This primarily
involves removing usages of deprecated methods including:

- Remove usages of Driver
- Use ChiselStage methods instead of BackendCompilationUtilities
  methods
- Use Dependency API for custom transforms
- Use extractCause to unpack StackError

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

Adds a new method, chiselTests.Util.containsCause, that will search
for a polymorphic exception anywhere in a stack trace. This is useful
if exceptions may move around (e.g., if they are suddenly wrapped in a
StageError).

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

Co-authored-by: Albert Magyar <albert.magyar@gmail.com>

* Canonicalize construction of empty Decoupled

* Change signature after dev meeting discussion
* Make EmptyBundle private and final

* Add test case for Decoupled with no payload

* Apply suggestions from code review

Co-authored-by: Richard Lin <richard.lin@berkeley.edu>

Co-authored-by: Albert Magyar <albert.magyar@gmail.com>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: Adam Izraelevitz <azidar@gmail.com>
Co-authored-by: Richard Lin <richard.lin@berkeley.edu>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

* Closes #1385

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

This reverts commit d4a481ce.

Move LFSR16 to Compatibility Mode (Deprecated in 3.2)

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