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

git-svn-id: http://bullet.googlecode.com/svn@2666 08e121b0-ae19-0410-a57b-3be3395fd4fd
update serialization structures

git-svn-id: http://bullet.googlecode.com/svn@2667 08e121b0-ae19-0410-a57b-3be3395fd4fd