refactor(pumpkin-core): Cleanup creation of propagator conflict

[maartenflippo]

Provide a convenient way to return a PropagatorConflict from a propagator.

The following types are moved:

Type	Old location	New location
PropagationStatusCP	basic_types/propagation_status_cp.rs	engine/conflict.rs
PropagatorConflict	basic_types/propagation_status_cp.rs	engine/conflict.rs
Conflict	engine/state.rs	engine/conflict.rs
EmptyDomainConflict	engine/state.rs	engine/conflict.rs

Note that this move affects many files, as it updates module paths. The types in the table above are now all part of the pumpkin_core::state module in the public API.