ExprContainer with value semantics

[Krzmbrzl]

This is supposed to be an alternative (longer-term perhaps even replacement) for ExprPtr. It can be used to store expressions but contrary to ExprPtr, the new ExprContainer has value semantics. That is, copying the container actually copies the underlying expression. This makes things much easier to reason about and as a side-effect this fixes the const issue of ExprPtr which allows you to do

void func(const ExprPtr &ptr) {
  auto copy = ptr; // only copies _pointer_
  expand(copy);
}

which ends up modifying the original expression ptr was is pointing to. Hence, with ExprPtr we don't have any way to avoid accidental modification.

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Note: This PR is based on top of #589

[evaleev]

@Krzmbrzl would it make sense to do copy-on-mutable-access to save on copying in trivial cases?

[Krzmbrzl]

would it make sense to do copy-on-mutable-access to save on copying in trivial cases?

I don't have experience implementing something like this but it seems like that would significantly complicate the implementation, no? Also, I would really like to not give up on the implicit conversion to (const) Expr &(&) which is what almost all operations act on. Having a copy-on-write seems like it would require introducing something like ExprContainer::to_expr() and Expr::to_mutable_expr() where the former returns const Expr & and the latter returns a copy (which would then still have to be stored in its own container again) 🤔

EDIT: Or are you strictly speaking about the copies performed during construction of ExprContainer instances themselves?