1. make /// ... and //! ... and /** ... */ and /*! ... */ into sugar for #[doc = ...] attributes.
2. add a script in etc/ to help converting doc-attributes to doc-comments
3. add some functions to core::str to help with (1)
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.
