Synthesize constructors

[Calvin-L]

Currently Cozy uses the concretization functions produced by query synthesis to implement constructors on the data structure. However, the concretization functions are optmized for AST size, not for performance, so the constructor implementations can be extremely slow.

The code in high_level_interface.py should manage some additional query synthesis threads that optimize the concretization functions for runtime performance. The change will also likely need to add constructor information to impls.py.