TeX printing of fractions.

This enhances the printing of Mul() involving short and long fractions. An option `fold_short_frac` is added to the latex printing that prints short fraction such as `1/x` simply as `1/x` instead of `\frac{1}{x}` previously. This saves vertical space, e.g. for inline math. This option is `True` by default, but I wouldn't mind making it `False` by default. The other improvement in this PR is to split long fractions into parts such as `\frac{p1}{q} (p2)`, where `p1` is a coeff and `p2` is e.g. an `Add`, instead of `\frac{p1 (p2)}{q}` previously.