printing imaginary unit as `im`?

[Roger-luo]

Hi I'm wondering if it is possible to print imaginary unit as im which is more consistent with Julia, the current one I is sometimes confusing with the LinearAlgebra.I.

[isuruf]

We already have specializations for NaN, Infty and E for Julia in https://github.com/symengine/symengine/blob/master/symengine/printers/strprinter.cpp. We should fix I as well.

[certik]

A related issue for parsing: symengine/symengine#1462