Changes:
- initializer expressions of constexpr variable are now wraped in a ConstantExpr. this is mainly used for testing purposes. the old caching system has not yet been removed.
- Add all the missing Serialization and Importing for APValue.
- Cleanup leftover from last patch.
- Add Tests for Import and serialization.
Why are you getting rid of this? It seems like we would still want these cleaned up.