First version of `symmip`, which adds tentative MIP support

[tBuLi]

First version of symmip, a symbolic wrapper around MIP solvers. Currently only support the Gurobi API, but the code is setup in such a way that we can easily add other solvers.

Added as a seperate module, because merging it into the existing symfit infrastructure was less than obvious. In particular, the existing Models, objectives and minimizers are only very expensive boilerplate with no added value for MIPs, so for now I cut ties with this formalism. We can always still plug this MIP solver back into the Fit API if we have a good MIP API to begin with.