[WIP]: remove Polynomial::Product, add Pair functor
An exercise. pairp is probably still faulty, as examples/dfa fails with
examples/dfa:16:6:
   |
16 | {f,n}xX^{a,b}
   |      ^
unexpected 'x'
expecting '#', '+', newline, or space
An exercise. pairp is probably still faulty, as examples/dfa fails with
examples/dfa:16:6:
   |
16 | {f,n}xX^{a,b}
   |      ^
unexpected 'x'
expecting '#', '+', newline, or space