Improve printing of symmetric matrices when used in constraints

[odow]

Closes #3764

So there is an argument for this, especially if we merge #3766, because it will help distinguish when JuMP exploits symmetry.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.38%. Comparing base (886a652) to head (ebb8b4d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3768   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          44       44           
  Lines        5879     5885    +6     
=======================================
+ Hits         5784     5790    +6     
  Misses         95       95           

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[odow]

What about some other options:

julia> c = @constraint(model, x == 0)
[x[1,1]  x[1,2]  x[1,3]  x[1,4]
 ↙       x[2,2]  x[2,3]  x[2,4]
 ↙       ↙       x[3,3]  x[3,4]
 ↙       ↙       ↙       x[4,4]] ∈ Zeros()

julia> c = @constraint(model, x == 0)
[x[1,1]  x[1,2]  x[1,3]  x[1,4]
 ∘       x[2,2]  x[2,3]  x[2,4]
 ∘       ∘       x[3,3]  x[3,4]
 ∘       ∘       ∘       x[4,4]] ∈ Zeros()

julia> c
[x[1,1]  x[1,2]  x[1,3]  x[1,4]
 ⋰       x[2,2]  x[2,3]  x[2,4]
 ⋰       ⋰       x[3,3]  x[3,4]
 ⋰       ⋰       ⋰       x[4,4]] ∈ Zeros()

julia> c
[x[1,1]  x[1,2]  x[1,3]  x[1,4]
 ⤯       x[2,2]  x[2,3]  x[2,4]
 ⤯       ⤯       x[3,3]  x[3,4]
 ⤯       ⤯       ⤯       x[4,4]] ∈ Zeros()

julia> c
[x[1,1]  x[1,2]  x[1,3]  x[1,4]
 ⦸       x[2,2]  x[2,3]  x[2,4]
 ⦸       ⦸       x[3,3]  x[3,4]
 ⦸       ⦸       ⦸       x[4,4]] ∈ Zeros()

julia> c
[x[1,1]  x[1,2]  x[1,3]  x[1,4]
 ⇗       x[2,2]  x[2,3]  x[2,4]
 ⇗       ⇗       x[3,3]  x[3,4]
 ⇗       ⇗       ⇗       x[4,4]] ∈ Zeros()

julia> c
[x[1,1]  x[1,2]  x[1,3]  x[1,4]
 ⋯       x[2,2]  x[2,3]  x[2,4]
 ⋯       ⋯       x[3,3]  x[3,4]
 ⋯       ⋯       ⋯       x[4,4]] ∈ Zeros()

[odow]

Use of \cdot was added in #1857 without discussion. I think we can use ⋯ (\cdots) for now.

It hints at "something else is here" and avoids confusion with ⋅ and 0.0.

[odow]

Merging because I think this is a win, and it is needed for #3766