This first patch starts by moving around pieces of state related to
type checking. The goal is to slowly unify the type checking state
into a single typing context. This initial patch moves the
ParameterEnvironment into the InferCtxt and moves shared tables
from Inherited and ty::ctxt into their own struct Tables. This
is the foundational work to refactoring the type checker to
enable future evolution of the language and tooling.

Use camel-case naming, and use names which actually make sense in modern Rust.

Helps prevent mixed content warnings if accessing docs over HTTPS.

Closes #25459

Functional changes for associated constants. Cross-crate usage of associated constants is not yet working.

Introduces new variants and types in syntax::ast, middle::ty, and middle::def.

`s/([^\(\s]+\.)len\(\) [(?:!=)>] 0/!$1is_empty()/g`

Closes #23909

Now that support has been removed, all lingering use cases are renamed.

`[]` on maps to `get` in rustc, since stage0 and stage1+ disagree about
how to use `[]`.

The stability check checks the `PublicItems` map when giving errors if
there is a #[stable] item with a public contents that doesn't not have
its own stability. Without recording this, struct fields and enum
variants will not get errors for e.g. stable modules with unmarked
functions internally.

This is just improving the compiler's precision to give the standard
library developers more information earlier.

E.g.

    #![staged_api]
    #![feature(staged_api)]
    #![crate_type = "lib"]

    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Foo {
        pub x: i32
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    pub mod bar {
        pub fn baz() {}
    }

Without the patch it gives:

    test.rs:12:5: 12:20 error: This node does not have a stability attribute
    test.rs:12     pub fn baz() {}
                   ^~~~~~~~~~~~~~~
    error: aborting due to previous error

With the patch it gives:

    test.rs:7:9: 7:15 error: This node does not have a stability attribute
    test.rs:7     pub x: i32
                      ^~~~~~
    test.rs:12:5: 12:20 error: This node does not have a stability attribute
    test.rs:12     pub fn baz() {}
                   ^~~~~~~~~~~~~~~
    error: aborting due to 2 previous errors