Refactor JuMP.jl, moving methods into their related files

[odow]

The diff is fairly large, but this has always annoyed me: The previous content of JuMP.jl was confusing. It wasn't ordered, and it contained a mix of methods that are better off living in other files.

For example, why was index(cr::ConstraintRef) in JuMP.jl, but index(v::VariableRef) in variables.jl?

There were also things like result_count(model) was in optimizer_interface.jl, but termination_status(model) was in JuMP.jl.

[codecov]

Codecov Report

Base: 98.06% // Head: 98.06% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (cba5104) compared to base (67ed280).
Patch coverage: 95.37% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3188      +/-   ##
==========================================
- Coverage   98.06%   98.06%   -0.01%     
==========================================
  Files          33       33              
  Lines        4592     4590       -2     
==========================================
- Hits         4503     4501       -2     
  Misses         89       89              

Impacted Files	Coverage Δ
src/mutable_arithmetics.jl	88.34% <ø> (ø)
src/operators.jl	96.31% <83.33%> (-0.33%)	⬇️
src/JuMP.jl	96.77% <91.66%> (+0.66%)	⬆️
src/optimizer_interface.jl	95.87% <95.91%> (-0.21%)	⬇️
src/constraints.jl	96.51% <100.00%> (+0.01%)	⬆️
src/variables.jl	100.00% <100.00%> (ø)

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[blegat]

I agree, there were many things in JuMP.jl that should have been at other places