Naming consistency

[odow]

I came across an annoying inconsistency developing MathOptFormat. Some structs use variable::VariableIndex:
https://github.com/JuliaOpt/MathOptInterface.jl/blob/97c92db2abb05ac8a53cedab0c4a68cc72d2fcf7/src/functions.jl#L40-L42
and other structs use variable_index::VariableIndex
https://github.com/JuliaOpt/MathOptInterface.jl/blob/97c92db2abb05ac8a53cedab0c4a68cc72d2fcf7/src/functions.jl#L64-L67

Thoughts on standardizing to variable? Because I can see people saying "Why?"

As a con, this is a horribly breaking change...

[mlubin]

Standardizing on variable seems reasonable. Post 0.19 though.