Tracking issue for num-complex 0.3

[cuviper]

• Update MSRV to 1.31, needed to deal with rand's use of package renaming
  • Update to 2018 edition and clippy fixes #71: Convert to 2018 edition too
• Arithmetic operations with reference semantics #35: Consider implementing operators by reference with NumRef etc.
  • Postponed.
• Improve MulAssign and DivAssign #50: Consider implementing assign-operators with internal assign-ops too.
• Upgrade to Rand 0.7 #65: Update to rand 0.7 (MSRV 1.32)

Other PRs and issues may be included too, but these are the ones that impact compatibility.