Fix constant propagation for unsynthesized blocks

[mdko]

When pyrtl.optimize() is called on unsynthesized blocks (i.e. blocks with multibit wires and all the higher-level nets), pyrtl.synthesize() fails during constant propagation.

For example:

a = pyrtl.Const(0b0101, 4, name='a')
a_inv = ~a
pyrtl.probe(a_inv, 'a_inv')

pyrtl.optimize()

results in

Traceback (most recent call last):
  File "const-prop-test.py", line 20, in <module>
    pyrtl.optimize()
  File "/Users/michael/PyRTL/pyrtl/passes.py", line 53, in optimize
    constant_propagation(block, True)
  File "/Users/michael/PyRTL/pyrtl/passes.py", line 182, in constant_propagation
    _constant_prop_pass(block, silence_unexpected_net_warnings)
  File "/Users/michael/PyRTL/pyrtl/passes.py", line 269, in _constant_prop_pass
    constant_prop_check(a_net)
  File "/Users/michael/PyRTL/pyrtl/passes.py", line 261, in constant_prop_check
    replace_net_with_const(output)
  File "/Users/michael/PyRTL/pyrtl/passes.py", line 210, in replace_net_with_const
    new_const_wire = Const(bitwidth=1, val=const_val, block=block)
  File "/Users/michael/PyRTL/pyrtl/wire.py", line 626, in __init__
    num, bitwidth = infer_val_and_bitwidth(val, bitwidth, signed)
  File "/Users/michael/PyRTL/pyrtl/helperfuncs.py", line 663, in infer_val_and_bitwidth
    return _convert_int(rawinput, bitwidth, signed)
  File "/Users/michael/PyRTL/pyrtl/helperfuncs.py", line 703, in _convert_int
    raise PyrtlError('insufficient bits for negative number')
pyrtl.pyrtlexceptions.PyrtlError: insufficient bits for negative number

The reason is two-fold:

1. The function being used to invert a constant number is incorrect (it was doing 1-x, but if you take the number 0b0101 (5) as an example, 1 - 5 = -4 = 0b1100 != 0b1010); please correct me if I'm wrong in my understanding of what was going on there.
2. The size of the constant being created in its place was always 1 (i.e. it was assuming synthesis had been done), rather than the size of the destination wire it was being fed to.

This PR fixes both of those things and adds a unit test. I think more unit tests should be added to test the correctness of optimization in general, but for expediency's sake, I've added at least one here.