refactor constraint serialization, so that double precision is maintained,
changes are backwards compatible (btBulletWorldImporter can load old .bullet files) but not forwards compatible (constraints in new .bullet files are ignored/unrecognized by old Bullet SDK) This commit is for Issue 734. Some more work needs to be done for btGImpactMeshShapeDoubleData and thus btStridingMeshInterfaceDoubleData and btMeshPartDoubleData
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