Explicitly initialize firrtl.stage.Forms to prevent multi-thread collisions (#1463)

* Explicitly initialize firrtl.stage.Forms to prevent multi-thread collisions
See https://github.com/freechipsproject/firrtl/issues/1462.
Convert `lazy val` members of firrtl.stage.Forms to plan `val`s.
Reference firrtl.stage.Forms in sufficient locations to ensure the object is initialized before its members are accessed.

* Respond to comments - make _dummyForms private.

* Move Forms initialization to package object.

* Merge with master