2.83
Add serialization support for btGeneric6DofSpring2Constraint Attempt to re-order serialization data so it can be serialized in-memory (for deep copy)
add the 'extras' and Bullet 2 tests, to make it easier to create a new intermediate release
fix a 64-bit issue in the optional 'preSwapFilenameOut' method (this is only use when you want to export a copy of a .bullet file after loading with a different endian-ness)