Speed up Puiseux series subroutine _new_polynomial().
Speed up Puiseux series subroutine _new_polynomial().

Consistently, the _new_polynomial() subroutine ran slowly, even
after gradyw's quickDiv addition. After some investigation it
appeared that the reduction of new generators (obtained via
functional composition and expression substitution) to the
generators [X,Y] took a long time. The current technique relies
on manually computing the new monomials after the appropriate
substutions to each monomial of the old polynomial. The generators
are always [X,Y], in this case.

Future speed-ups can focus on appropriate handling of the RootOf
symbolics. Perhaps we can pass around and generate the new base
fields.

abelfunctions/abelfunctions,abelfunctions/abelfunctions,cswiercz/abelfunctions,cswiercz/abelfunctions,cswiercz/abelfunctions,abelfunctions/abelfunctions