Bugfix for reified constraint

[Wikunia]

Solved #202

There were some pruning possibilities missing in the reified constraint as well as an error in how still_feasible was implemented.

In particular the following things were added:

• if inner constraint can't be activated it shouldn't be solved
• if the inner constraint is anti-solved (all fixed but don't fulfill) the reified variable can be set to !activate_on

[codecov]

Codecov Report

Merging #204 (61f461c) into master (a4420cb) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   96.67%   96.77%   +0.10%     
==========================================
  Files          36       36              
  Lines        3066     3072       +6     
==========================================
+ Hits         2964     2973       +9     
+ Misses        102       99       -3     

Impacted Files	Coverage Δ
src/constraints/reified.jl	100.00% <100.00%> (ø)
src/ConstraintSolver.jl	99.34% <0.00%> (+0.65%)	⬆️
src/constraints/not_equal.jl	100.00% <0.00%> (+2.38%)	⬆️

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