Add InlineInstance API

This adds a new trait, InlineInstance, to chisel3.util.experimental. This
trait, when mixed into a specific module or instance, will "inline" that
module, i.e., "collapse a module while preserving it's submodules."

This includes testing (InlineSpec) and ScalaDoc documentation.

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