Make specialized power::expand() helpers static member fuctions.

Make specialized power::expand() helpers static member fuctions.

The class power member functions expand_add(), expand_add_2(), and
expand_mul() do not access any member variable of class power. (In
fact, the only reason not to hide them entirely is that they invoke
protected members of classs expairseq, add, and mul which requires
them to be friends of these classes.)