faster ``minpoly_pow`` using resultants
faster ``minpoly_op_algebraic_number`` with co-prime degrees
added option ``compose`` to ``minpoly`` to use ``minpoly1``

``minpoly1`` computes the minimal polynomial using arithmetic operations
on minimal polynomials.

Added ``minpoly_add``, ``munpoly_mul``.

Added ``_minpoly_sin``, ``_minpoly_cos`` and ``_minpoly_exp`` to
compute the minimal polynomial of trigonometric expressions.
This implementation is incomplete;
the case with angles ``pi*p/q`` with ``q`` prime has been implemented.