Replace bit-oriented 16x16 multiply with routine using MULWF.

Existing routine worked correctly but was using shift and add
instructions suitable for the PIC16 family, from whence it came.
New version uses 8x8 MULWF instruction available on PIC18.
This also means smul.asm and smul.inc are obsolete and removed
from the project and the LKR file.  Will eventually be deleted
from the repository.