Counting the number of times equations hold

[dourouc05]

Follow-up from jump-dev/MathOptInterface.jl#846.

In standard Julia, you can compare arrays of numbers, and see the number of equalities that hold:

t = 1
σ = 2
ρ = 1
β = 4
[t, σ] .== [ρ, β] # BitVector: [true, false]
sum([t, σ] .== [ρ, β]) # 1: one of the two equalities hold

However, the closest syntax with Symbolics does not work yet:

@parameters t σ ρ β
sum([t, σ] .~ [ρ, β])

It seems that it's trying to sum the equations side by side:

ERROR: MethodError: no method matching +(::Equation, ::Equation)
Closest candidates are:
  +(::Any, ::Any, ::Any, ::Any...) at operators.jl:560
  +(::ChainRulesCore.AbstractThunk, ::Any) at C:\Users\Thibaut\.julia\packages\ChainRulesCore\GciYT\src\differential_arithmetic.jl:138
  +(::ChainRulesCore.Tangent{P, T} where T, ::P) where P at C:\Users\Thibaut\.julia\packages\ChainRulesCore\GciYT\src\differential_arithmetic.jl:162
  ...
Stacktrace:
  [1] add_sum(x::Equation, y::Equation)
    @ Base .\reduce.jl:24
  [2] _mapreduce
    @ .\reduce.jl:408 [inlined]
  [3] _mapreduce_dim
    @ .\reducedim.jl:318 [inlined]
  [4] #mapreduce#672
    @ .\reducedim.jl:310 [inlined]
  [5] mapreduce
    @ .\reducedim.jl:310 [inlined]
  [6] #_sum#682
    @ .\reducedim.jl:878 [inlined]
  [7] _sum
    @ .\reducedim.jl:878 [inlined]
  [8] #_sum#681
    @ .\reducedim.jl:877 [inlined]
  [9] _sum
    @ .\reducedim.jl:877 [inlined]
 [10] #sum#679
    @ .\reducedim.jl:873 [inlined]
 [11] sum(a::Vector{Equation})
    @ Base .\reducedim.jl:873
 [12] top-level scope
    @ REPL[13]:1

Similarly, it would be nice to allow count, the simplest usage being very similar:

julia> count([t, σ] .~ [ρ, β])
ERROR: TypeError: non-boolean (Equation) used in boolean context
Stacktrace:
 [1] _simple_count
   @ .\reduce.jl:970 [inlined]
 [2] _count
   @ .\reducedim.jl:395 [inlined]
 [3] #count#676
   @ .\reducedim.jl:393 [inlined]
 [4] #count#675
   @ .\reducedim.jl:392 [inlined]
 [5] count(A::Vector{Equation})
   @ Base .\reducedim.jl:392
 [6] top-level scope
   @ REPL[4]:1```



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