Use cpp11

[mgirlich]

• how to deal with memcpy()?
• is the performance hit of the string handling relevant?
• xml2_node.cpp can be tackled after #400 is merged.

[mgirlich]

@DavisVaughan I now also migrated xml2_node.cpp to cpp11. But the performance hit from string handling really bites us now. It is nearly as slow as before replacing the S3 dispatch.

[DavisVaughan]

Is it one or two functions in particular?

[mgirlich]

I haven't looked into this in more detail but only checked xml_name() (see the benchmark in https://github.com/r-lib/xml2/pull/400). The easiest would probably be to check and carefully revert some of the changes and then wrap things with cpp11::unwind_protect().