Add Node.copyExtraInfoTo(Node other), use in copy()

This way, if the extraInfo interface changes in the future (see
discussion in #17), I’ll only need to update Node and not the dozens of
copy() methods as well.

Done with the following command:

    find source/ceylon/ast/core/ -name '[A-Z]*.ceylon' \
        -exec \
            sed -i \
            -e 's|ret\.extraInfo = extraInfo|copyExtraInfoTo(ret)|' \
        {} +