Refactor Annotations (#767)

* Generalize ChiselAnnotation

This allows us to delay creation of Annotations till elaboration is
complete. Also update all annotation-related code.

* Add RunFirrtlTransform

Use a Chisel-specific RunFirrtlTransform API to preserve behavior of old
ChiselAnnotation (now called ChiselLegacyAnnotation)

* Use unique test directories in ChiselRunners.compile