bin variable symbol as constraint

[Wikunia]

Implementation for #250
Currently it does not check whether the variables used are binary as I'm unsure how to do this.

[codecov]

Codecov Report

Merging #267 (f8f1bd4) into master (29ea78a) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
+ Coverage   97.53%   97.58%   +0.05%     
==========================================
  Files          53       53              
  Lines        4419     4428       +9     
==========================================
+ Hits         4310     4321      +11     
+ Misses        109      107       -2     

Impacted Files	Coverage Δ
src/MOI_wrapper/bool.jl	95.23% <100.00%> (+1.29%)	⬆️
src/ConstraintSolver.jl	99.14% <0.00%> (+0.56%)	⬆️

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[Wikunia]

Should probably allow

a := { b => {c}}

Edit: That doesn't seem to work here as c is a normal expr and I don't have access at the parse constraint call in that case