Rename m field of VariableRef to model.

[mlubin]

For style compliance. Also switch to owner_model method instead of
accessing the field directly.

The definition of VariableRef is now:

struct VariableRef <: AbstractVariableRef
    model::Model
    index::MOI.VariableIndex
end

which is much nicer IMO than

struct VariableRef <: AbstractVariableRef
    m::Model
    index::MOIVAR
end

We should do the same for ConstraintRef after this goes through.

[codecov]

Codecov Report

Merging #1483 into master will increase coverage by 0.42%.
The diff coverage is 94.02%.

@@            Coverage Diff             @@
##           master    #1483      +/-   ##
==========================================
+ Coverage   88.12%   88.54%   +0.42%     
==========================================
  Files          27       27              
  Lines        3654     3668      +14     
==========================================
+ Hits         3220     3248      +28     
+ Misses        434      420      -14

Impacted Files	Coverage Δ
src/JuMP.jl	75.67% <100%> (+0.22%)	⬆️
src/parse_nlp.jl	87.67% <66.66%> (-0.61%)	⬇️
src/variables.jl	93.45% <94.64%> (ø)	⬆️
src/nlp.jl	84.95% <0%> (+2.83%)	⬆️

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