latex(Mul()): Don't special-case coeff.

Print x/2 as \frac{x}{2} instead of \frac{1}{2} x. This makes it
consistent with str and pretty printers and simplify the code
quite a bit.
Refine needs_mul_brackets.

Use expr.has() instead of isinstance to account for e.g.
Mul(2, Integral(x, x)) which needs brackets.