Support both MOI v0.9 and MOI v0.10

[ericphanson]

Builds off of #467

I think we can support both MOI v0.9 and MOI v0.10 pretty easily, since there are so few changes we need to make. It's a bit hacky, but I think it is worth it, since there might be a long tail of solvers that aren't updated. E.g. dropping MBP support in Convex was pretty annoying for me personally (even though I was the one who did it) since I ended up wanting to use Pajarito, and this requires much less compat shimming than that would have.

I've updated CI here to test with both versions.

[codecov]

Codecov Report

Merging #468 (f0eac7a) into master (c10461b) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #468      +/-   ##
==========================================
+ Coverage   91.82%   92.09%   +0.27%     
==========================================
  Files          83       83              
  Lines        5137     5137              
==========================================
+ Hits         4717     4731      +14     
+ Misses        420      406      -14     

Impacted Files	Coverage Δ
src/solution.jl	92.09% <100.00%> (+7.90%)	⬆️

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[odow]

Nice. If you want something that doesn't use try-catch, I think you should be able to use

if !isdefined(MOI, :SingleVariable) || MOI.SingleVariable isa Function
    const SV_OR_VI, SV_OR_IDENTITY = MOI.VariableIndex, identity
else
    const SV_OR_VI, SV_OR_IDENTITY = MOI.SingleVariable, MOI.VariableIndex
end

[ericphanson]

Thanks, that's nicer! (The second branch should have MOI.SingleVariable, MOI.SingleVariable, by the way). Made that change in f0eac7a; if CI passes I'll merge+tag.