experimental/debug-coercions-as-xml

We're all pretty helpless seeing messages like:

  value is an attribute set while a list was expected

This patch add the flag --xml-debug-coercion-failure
making nix output the xml representation of the attribute set.

Please not that this may fail if the thing to be coerced
cannot be turned into xml, eg because its evaluation does not end, eg due to
infinite recursion.

TODO: document this flag

Signed-off-by: Marc Weber <marco-oweber@gmx.de>