deprecate + on constraints

[ericphanson]

IMO it is kind of goofy, and doesn't match how the rest of Julia works which can make it confusing. There is also not much of a mathematical or translation purpose; I've never seen constraints written that way in a paper.

Also, I've updated the Problem constructors to be less picky about the types, which makes manual construction of the constraints vector (like constraints = []; push!(...) less annoying). I suspect the p = minimize(...); p.constraints += ... thing was partly to make the constructors work.

Lastly, I fixed a small bug: the field constraints was typed as an Array, which meant you could construct a Matrix{Constraint} as the constraints field in Problem, which would then break the new printing code. Here I've typed it as a Vector.

[odow]

Does this need a separate warning? You can still use vcat(x, y)

[ericphanson]

I thought it might be confusing to share an error, since someone doing x + y might not think of it that way

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.96%. Comparing base (68e9efc) to head (90016a6).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
+ Coverage   97.88%   97.96%   +0.07%     
==========================================
  Files          89       89              
  Lines        5205     5203       -2     
==========================================
+ Hits         5095     5097       +2     
+ Misses        110      106       -4     

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