Implement symbolic factorial

[sritchie]

This is such a nice one! sicmutils.special.factorial/factorial doesn't work with symbols out of the box, because you'd have to know... well, the value of 'x to generate all of the factors.

The task here is to:

• add sicmutils.numsymb/factorial, which will generate (factorial x)
• extend the renderers to render this as x!
• brainstorm and implement some simplification rules, where, say, x! / (x-2)! => x(x-1).

[adamhaber]

I'll try!