[FIRRTL] ConstCast: cast aggregates instead of ground elements of aggregates?

ConstCast nicely supports aggregate types, but presently we only const-cast individual leaves of aggregates, even when a higher-level const cast could do the trick.

Sample input:

circuit ConstAgg:
  module ConstAgg:
    input x: const { a: UInt<1>[100] }
    output y: { a: UInt<1>[100] }

    y <= x

Running with --preserve-aggregate=all --scalarize-top-module=false, we const-cast + connect each of the 100 individual elements.

This is done for uninferred reset/width casts, as they do not support aggregate types (yet?).

It'd be nice to emit the higher-level constcast directly, to preserve aggregates, if possible to do without excessive recursive checking.

Locate the FIRRTL ConstCast implementation and reproduce the sample with --preserve-aggregate=all and --scalarize-top-module=false. Compare the current per-leaf casts with the desired aggregate-level cast; done means preserving the aggregate while retaining correct behavior for uninferred reset/width casts.