Rename LaurentPolynomial_generic to LaurentPolynomial (it's not a "generic" implementation, it's an abstract base class) and put some abstract methods there, like cpdef _add_.

CC: @roed314

Component: basic arithmetic

Author: Jeroen Demeyer

Branch/Commit: b0cee8a

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/23606