Feat: heuristic rule might result in already generated expr making expr replacement tricky

[AveryQi115]

Context

#88

Current Design

When the generated expr by heuristic rule already exist in another group, we merge two groups and mark the old expr as a dead end (mark all rules have been fired for it.) It might be costly.

And here are several alternatives:

1. remove the old expr, but keep the group so that its parent can still find it. (might lead to an empty group without expr, might cause future problems)
2. add a flag to mark the expr is a dead end

Test Guidance

It happens in two scenarios:

1. the generated expr by heuristic rule already exist in another group
2. the generated expr is a placeholder type containg a new group id

With small number of rules, the first scenario is quite rare. The feat might be improved and tested with more rules in the future.