Change method lookup to require invariance for mutable references

Fixes #17033.
Fixes #15965.
Document why `core::str::Searcher::new` doesn't overflow
librustc: Forbid inherent implementations that aren't adjacent to the
type they provide an implementation for.

This breaks code like:

    mod foo {
        struct Foo { ... }
    }

    impl foo::Foo {
        ...
    }

Change this code to:

    mod foo {
        struct Foo { ... }

        impl Foo {
            ...
        }
    }

Additionally, if you used the I/O path extension methods `stat`,
`lstat`, `exists`, `is_file`, or `is_dir`, note that these methods have
been moved to the the `std::io::fs::PathExtensions` trait. This breaks
code like:

    fn is_it_there() -> bool {
        Path::new("/foo/bar/baz").exists()
    }

Change this code to:

    use std::io::fs::PathExtensions;

    fn is_it_there() -> bool {
        Path::new("/foo/bar/baz").exists()
    }

Closes #17059.

RFC #155.

[breaking-change]
auto merge of #17130 : jakub-/rust/issue-17033, r=pcwalton

Fixes #17033.
Fixes #15965.

cc @nikomatsakis
auto merge of #17163 : pcwalton/rust/impls-next-to-struct, r=alexcrichton

type they provide an implementation for.

This breaks code like:

    mod foo {
        struct Foo { ... }
    }

    impl foo::Foo {
        ...
    }

Change this code to:

    mod foo {
        struct Foo { ... }

        impl Foo {
            ...
        }
    }

Closes #17059.

RFC #155.

[breaking-change]

r? @brson
auto merge of #17195 : tbu-/rust/pr_strfindoverflow2, r=alexcrichton
Fix regression and overflow bug for rationals.