copy() within txn() is not transactional

[mohit10verma]

If we use copy() within txn(), the store will happen, but the old values will not be logged before the update.
Go compiler replaces copy with a runtime call very early during compilation. So during ssa creation, there is no easy way to identify a node which was originally "OCOPY" node.

Ideally go-pmem needs to support transactional copy() semantics within a txn() block OR
throw a syntax error if user uses copy() within txn() block.