Smallintmap was being populated with sparse keys during ast deserialization.

Impls can implement either zero or one traits; this has been true
more or less since we removed classes. So I got rid of the comments
saying "we should support multiple traits" and changed the code to
make it clear that we don't. This is just cleanup, and doesn't break
any existing tests.

but do not integrate with kindck etc (requires a snapshot first)

Replace all instances of #[auto_*code] with the appropriate #[deriving] attribute
and remove the majority of the actual code, leaving stubs to refer the user to
the new syntax.

fail!() used to require owned strings but can handle static strings
now. Also, it can pass its arguments to fmt!() on its own, no need for
the caller to call fmt!() itself.

its own type. Use a bitset to represent built-in bounds. There
are several places in the language where only builtin bounds (aka kinds)
will be accepted, e.g. on closures, destructor type parameters perhaps,
and on trait types.

They're still parsed though, to get through bootstrapping.

Fixes #6141.