Make EmittedAnnotation a SingleTargetAnnotation

Change EmittedAnnotation from a NoTargetAnnotation to a
SingleTargetAnnotation. Concretely, this means that some string that
the emitter produces, be it a Verilog or FIRRTL IR string, is now
associated with a specific circuit "thing". This has the benefit of
allowing for transforms that run after an emitter to know which
emitted annotation is associated with a specific component.

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