Update MOI interface

[klamike]

@blegat thanks for your suggestions in #25, would super appreciate your review here.

• Implements default_cache with UniversalFallback (I was getting some error with MOI.Silent otherwise)
• Renames to src/MOI_wrapper.jl
• Adds "Use with JuMP" section to README
• Adds environment variable to control when to run MOI test suite

closes #25

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[gdalle]

Not an expert on MOI bindings so I have a few surface comments but I'll let @blegat validate the rest if he has time (otherwise I trust @klamike's judgement)

[gdalle]

I just realized we don't need a separate workflow file for the MOI tests. We can get away with a strategy matrix that sets the environment variable (check out how I did it in DifferentiationInterface)

[klamike]

@blegat looking good now?

[blegat]

Looks good :)

[klamike]

@gdalle is this how you wanted the CI set up?

[gdalle]

Yes, perfect, this will speed things up a bit by parallelizing