fix: avoid deep copying unnecessarily in current

[francescotescari]

Fix for #47

Avoids creating a copy of all the nested objects when invoking current by lazily computing the copy of non-draft objects, only if necessary.

Performance seems to be still worse than just create (aka finalize), but at least it avoids creating new objects when unnecessary.

[unadlib]

hi @francescotescari , thank you for submitting the PR to improve performance.

Although using Symbol for marking is ok, I think it would be more appropriate to directly use object references to handle it. Nevertheless, I'm very grateful for your PR, and I will merge it.

[francescotescari]

Although using Symbol for marking is ok, I think it would be more appropriate to directly use object references to handle it.

Good point. I was blind not seeing that :D
Opened a follow up if you mind