Generalize what was formerly the unsigned-only architecture to support
both signed and unsigned multiplication, use that as default, and set
aside the special low-power architecture that was formerly used for
signed multipliers.
In commit fedd12261 ("booth: Move away from explicit `Wire` pointers")
a bug was introduced when checking for vacant slots in arrays holding
some intermediate results. Non-wire SigBit values were taken to imply
a vacant slot, but actually a constant one can make its way into those
results, if the multiplier cell configuration is just right. Fix the
vacancy check to address the bug.
For the basic single-bit operations, opt for gate cells (`$_AND_` etc.)
instead of the coarse cells (`$and` etc.). For the emission of cells
move to the conventional module methods (`module->addAndGate`) away
from the local helpers. While at it, touch on the surrounding code.
To represent intermediate signals use the `SigBit`/`SigSpec` classes as
is customary in the Yosys codebase. Do not pass around `Wire` pointers
unless we have special reason to.
