[FIRRTL] extmodule's supporting enablelayer?

[dtzSiFive]

FIRRTL spec only covers this for modules, and use on extmodule's in untested and unverified.

The following parses and compiles fine:

FIRRTL version 4.0.0

circuit EL:
  extmodule X enablelayer A.B.C :
  public module EL :

Supporting this allows extmodule to describe a module with enablelayers such that it's instantiated appropriately. Assuming we want this, spec should be updated and verifier added accordingly.