Signed comb with unsigned table

Signed comb with unsigned table

Or, bitwiseshiftleft saves the day.  The current code is hacky as hell,
but it works, and it cleared up my confusion.  Turns out a suitable
signed comb is quite different from an unsigned one: the table itself
should represent -1 and 1 bits, instead of 0 and 1 bits.

Right now the same effect is achieved with 2 additions (more precisely,
an addition and a subtraction).  With the proper table, it can be one
operation.