Add FlattenInstance API

This adds a new trait, FlattenInstance, to chisel3.util.experimental. When
mixed into a module or a specific instance this trait will "flatten",
i.e., "inline that module and all of its submodules".

This includes testing (additions to InlineSpec) and ScalaDoc
documentation.

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