use SAT encoding of fused-multiply-add

this may speed up things because of smaller formulas. Example: binary with overflow bit, total width 3 (so, values 0,1,2,3,large):

• times: 7 NAND nodes with 24 edges
• plus: 7 NAND nodes 20 edges
• fma: 12 NAND nodes with 43 edges

plan:

• introduce method with default implementation

class Semiring s where
  fma :: s ->  s -> s -> s
  fma x y z = plus (times x y) z

• use that method in matrix multiplication
• overwrite default impl. for selected types