basic indicator in reified constraint

[Wikunia]

Partly implements #248

such that this:

@constraint(model, m7 := { m3 => {m6 == 1}})

works. Would work nicer with #250

Todo:

• Make sure that m7 := { m3 => {m6 > 1}} fails early as unsupported or do actually support it 😄
• Test case for reified in reified, indicator in indicator, reified in indicator

@hakank I used your http://hakank.org/julia/constraints/monks_and_doors.jl as a test case for this.
Feel free to double check this with other models that you have.

[codecov]

Codecov Report

Merging #251 (a06baf4) into master (3e11d24) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
- Coverage   97.57%   97.53%   -0.04%     
==========================================
  Files          53       53              
  Lines        4408     4419      +11     
==========================================
+ Hits         4301     4310       +9     
- Misses        107      109       +2     

Impacted Files	Coverage Δ
src/MOI_wrapper/util.jl	100.00% <100.00%> (ø)
src/constraints/indicator.jl	100.00% <100.00%> (ø)
src/constraints/reified.jl	100.00% <100.00%> (ø)
src/ConstraintSolver.jl	98.57% <0.00%> (-0.57%)	⬇️

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[Wikunia]

Decided to not support bridges for now as reified and indicator bridges are not SetMapBridge atm and I think this is a relatively minor thing.