Compatibility import for variable changes

The LeanPool project already contains the exact point-group change-of-variables API needed by the reduction cone, ported from Michael Stoll's Apache-2.0 EllipticCurves project at commit 3f8c39c0fc4c0fd0a40e693aa2a9bbda08d9ee1f.

Keeping this historical module name as a public import lets the Stoll reduction files retain their original import graph without redeclaring the shared WeierstrassCurve.Affine.Point.equivVariableChange API.