fix(library/tactic): memory leaks

Proof/Cex builders and tactics implemented in Lua had a "strong reference" to script_state. If they are stored in the Lua state, then we get a cyclic reference.
That is, script_state points to these objects, and they point back to script_state.

To avoid this memory leak, this commit defines a weak reference for script_state objects. The Proof/Cex builders and tactics now store a weak reference to the Lua state.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>