Improve printing of certain expressions (e.g. omit trivial coefficients, avoid redundant parenthesis etc.)

[fingolfin]

Currently we have things like this:

(1) * exp(2π𝑖(2//(q - 1)*i*k))
(q) * exp(2π𝑖(2//(q - 1)*i*k))

which really should be rendered as

exp(2π𝑖(2//(q - 1)*i*k))
q * exp(2π𝑖(2//(q - 1)*i*k))

We can try to do that manually. Alternatively, perhaps consider using show_via_expressify (from AbstractAlgebra) to improve some of the printing systematically (it was designed for just this kind of problem).