Improve printing of complex numbers

[blegat]

I tried to make it consistent with both how Base prints complex numbers (im is just on the right of the imaginary part without any * in between) and how JuMP prints real coefficients.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (adad825) 98.05% compared to head (39325ef) 98.06%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3332      +/-   ##
==========================================
+ Coverage   98.05%   98.06%   +0.01%     
==========================================
  Files          34       34              
  Lines        4876     4911      +35     
==========================================
+ Hits         4781     4816      +35     
  Misses         95       95              

Impacted Files	Coverage Δ
src/print.jl	99.76% <100.00%> (+0.02%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[odow]

If we only have Float64 and Complex, then this makes more sense than what we had before. But I wonder if we should instead unify our coefficient printing and simplify: #3341.

[blegat]

I think it's nice to have at least optional rounding. Now we can discuss whether we want it by default and I think would be in favor

[odow]

I'll fix the conflict, since it's my fault for #3341.