Fix FIRRTL to Verilog process instance subfield assignment.

Fix FIRRTL to Verilog process instance subfield assignment.
Don't emit subfield assignments: bits(x, y, z) <= ... - but instead, add them to the reverse-wire-map where they'll be treated at the end of the module.
Enable tests which were disabled due to incorrect treatment of subfields.
Assume the `$firrtl2verilog` variable contains any additional switches to control verilog generation (i.e. `--no-dedup -X mverilog`)