Add --start-from option (#2273)

Add a new option to the FIRRTL compiler, "--start-from = <form>".  If
used, this will cause the compiler to assume that the input FIRRTL
circuit is already in the specific form.  It will then skip unnecessary
passes given this information.

E.g., if a user requests to run "firrtl -X verilog --start-from low"
then the compiler will only run transforms necessary to get from low
FIRRTL to Verilog.  Transforms necessary for ingesting FIRRTL IR will be
run if needed (checks and type/kind/flow resolution).

To implement this, a CurrentFirrtlStateAnnotation is added.  Advanced
users can use this directly to tell the FIRRTL compiler exactly what
transforms have already been run, including the ability to ignore checks
or type/kind/flow resolution if they so desire.

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