Rewrite C wrapper using Clang

[odow]

Getting this shipshape before I step back from maintaining it.

I don't know what the deprecation path is.

The biggest issue is the callback file, which had some constants CB_MIPSOL, and some functions like cbget_xxx. Is it worth deprecating those and nothing else?

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[odow]

Same deprecation strategy as CPLEX.

The C API of Gurobi.jl has been rewritten to expose the complete C API, and
all old functions have been removed.

For example:

    model = Gurobi.Optimizer()
    stat = Gurobi.get_status_code(model.inner)

is now:

    model = Gurobi.Optimizer()
    valueP = Ref{Cint}()
    ret = GRBgetintattr(model, "Status", valueP)
    if ret != 0
        # Do something because the call failed
    end
    stat = valueP[]

The new API is more verbose, but the names and function arguments are now
identical to the C API, documentation for which is available at:
https://www.gurobi.com/documentation/9.0/refman/c_api_details.html

To revert to the old API, use:

    import Pkg
    Pkg.add(Pkg.PackageSpec(name = \"Gurobi\", version = v\"0.8.1\"))

Then restart Julia for the change to take effect.

[odow]

This PR is a little hard to review, so I propose merging into master to make it easier for people to try out, with the view to waiting a while before tagging and releasing. Then any extra changes will be much easier to review.

[odow]

Merging with the same plan as CPLEX: jump-dev/CPLEX.jl#316 (comment)