This comes with a terminology change. All linkage-symbols are 'extern'
now, including rust syms in other crates. Some extern ABIs are
merely "foreign". The term "native" is retired, not clear/useful.
What was "crust" is now "extern" applied to a _definition_. This
is a bit of an overloading, but should be unambiguous: it means
that the definition should be made available to some non-rust ABI.
This involved adding 'copy' to more generics than I hoped, but an
experiment with making it implicit showed that that way lies madness --
unless enforced, you will not remember to mark functions that don't
copy as not requiring copyable kind.
Issue #1177
While it is still technically possible to test stage 0, it is not part of any
of the main testing rules and maintaining xfail-stage0 is a chore. Nobody
should worry about how tests fare in stage0.
