Export Multiplicities from GHC.Exts

Both Data.Kind and GHC.Exts export FUN, which takes a argument of kind Multiplicity. But to actually write out its kind or supply and argument of kind Multiplicity, one needs to import GHC.Prim.

Since the Multiplicity kind and it's Many and One constructors are explicitly part of the linear types proposal, they appear to be a user-facing design choice, rather than an implementation detail. Hence I propose exporting them from GHC.Exts as well.

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