feat(cp): Improve efficiency of linear constraints propagation

[arbimo]

• feat(cp): Reactivate watch-based propagation of linear constraints.
• chore(cp): Make watches on VarRef (less error prone than SVar)
• feat(cp): Only trigger propagation once per constraint.
• chore(cp): Clean up internal conversions from/to i64.

[arbimo]

@Yirmandias You may try this branch that should substantially reduce the propagation time of linear constraints.