Refactor MOI._constant and MOIU.getconstant

[blegat]

Create good names consistent with the JuMP style guide

[matbesancon]

@blegat there is __constant in Bridge, should this be merged too? It would require moving functions around, otherwise this would be type piracy from MOIB (it's implementing a MOI function over MOI types)

[blegat]

Yes, I mentioned in #723 (comment) (sorry as you can see, I didn't see your comment when writing this one), we can merge it with __constant and move the implementations to MOI