General layout + SSReflect import

SSReflect v 1.5 + libraries (poly/int/quotient + dependencies)
import
Import free group theory from EC + extend it to abstract coeffs (ring)
multinom def.
Import current state of the multinomials library.

This is a draft. Contains:

 - ring structure
 - injection in iterated polynomials
 - morphism
 - evaluation
 - symmetric