Support explicit FIRRTL properties for better accommodation of FIRRTL/Verilog semanti...

Support explicit FIRRTL properties for better accommodation of FIRRTL/Verilog semantic differences.
Use FIRRTL spec vlaues for definition of FIRRTL widths.
Added support for '$pos`, `$pow` and `$xnor` cells.
Enable tests/simple/operators.v since all operators tested there are now supported.
Disable FIRRTL tests of tests/simple/{defvalue.sv,implicit_ports.v,wandwor.v} since they currently generate FIRRTL compilation errors.