Expose Hindley–Milner inference engine as public API

[shingarov]

In the current codebase, our vulgar-type inference machinery is only used by the refinement-type engine internally. There are, however, other places in the Smalltalk-25 system (e.g. when typechecking Sail) where Hindley–Milner is needed. The present Issue calls to expose this functionality via an API usable from outside MachineArithmetic.