When we know we have the only reference to a Report or Node object we
can avoid copying the data to change it. Add "Unsafe" variants of
various Merge operations which mutate the receiver, and a new Merger
which takes advantage of them.
This is an alternate way of solving the same problem as 4007a902a264e5ff2c3be6b269ade515c9c1c145,
but in a nicer way. Compared to using pointers, this approach more obviously preserves the original
behaviour, and is arguably more readable than the original code.
Credit to @rade for this approach.
