Infinite recursion with %#v formatter

[Wessie]

Giving any of the fmt.*f functions a xmlpath.Node will result in an infinite recursion and an eventual stack overflow. This happens due to Node.nodes containing non-pointers and the recursive referencing with the nodes in it.

non-xmlpath reproduce
xmlpath reproduce

While the fmt package is the one at fault for not handling recursion. xmlpath might want to implement a GoString to avoid the stack overflow for users.

Reported it as a Go issue too as issue 8241. This mostly to get a fmt documentation update.