rootof: add rootofsimp() simplification function

rootofsimp() reduces rational expressions of RootOf types modulo the
RootOf's defining polynomial. rootofsimp performs this simplification
with each distinct RootOf appearing in the input expression.

This commit replaces a previous implementation where powers of RootOfs
were automatically simplified / reduced modulo their defining
polynomial.

Documentation for rootofsimp() is added to the simplify documentation
page.

Example:

>>> from sympy import rootofsimp, RootOf
>>> from sympy.abc import x,y
>>> r = RootOf(x**4 - x - 1, 0)
>>> rootofsimp(r**4)
RootOf(x**4 - x - 1, 0) + 1
>>> rootofsimp(r**8)
RootOf(x**4 - x - 1, 0)**2 + 2*RootOf(x**4 - x - 1, 0) + 1
>>> rootofsimp(1/r)
RootOf(x**4 - x - 1, 0)**3 - 1