librustc: Stop assuming that implementations and traits only contain
methods.

This paves the way to associated items by introducing an extra level of
abstraction ("impl-or-trait item") between traits/implementations and
methods. This new abstraction is encoded in the metadata and used
throughout the compiler where appropriate.

There are no functional changes; this is purely a refactoring.
auto merge of #16286 : pcwalton/rust/associated-items-groundwork, r=nikomatsakis

methods.

This paves the way to associated items by introducing an extra level of
abstraction ("impl-or-trait item") between traits/implementations and
methods. This new abstraction is encoded in the metadata and used
throughout the compiler where appropriate.

There are no functional changes; this is purely a refactoring.

r? @nick29581
librustc: Forbid external crates, imports, and/or items from being
declared with the same name in the same scope.

This breaks several common patterns. First are unused imports:

    use foo::bar;
    use baz::bar;

Change this code to the following:

    use baz::bar;

Second, this patch breaks globs that import names that are shadowed by
subsequent imports. For example:

    use foo::*; // including `bar`
    use baz::bar;

Change this code to remove the glob:

    use foo::{boo, quux};
    use baz::bar;

Or qualify all uses of `bar`:

    use foo::{boo, quux};
    use baz;

    ... baz::bar ...

Finally, this patch breaks code that, at top level, explicitly imports
`std` and doesn't disable the prelude.

    extern crate std;

Because the prelude imports `std` implicitly, there is no need to
explicitly import it; just remove such directives.

The old behavior can be opted into via the `import_shadowing` feature
gate. Use of this feature gate is discouraged.

This implements RFC #116.

Closes #16464.

[breaking-change]