nodes in statement position.

Extended #[cfg] folder to allow removal of statements, and
of expressions in optional positions like expression lists and trailing
block expressions.

Extended lint checker to recognize lint levels on expressions and
locals.

This improves bootstrap times because of better parallelism - though I need to measure how much - and allows metadata to be modified without triggering a full recompile. This also ensures that metadata handling and the rest of rustc remain decoupled, which is a first step for switching to a new metadata format.

This is a [breaking-change] to all plugin authors because of the following renames:
 * `rustc::plugin` is now `rustc_plugin`
 * `rustc::metadata` is now `rustc_metadata`
 * Most data types from `rustc::metadata`, along with `LOCAL_CRATE`, are now in `rustc::middle::cstore`.
 * The CStore methods were split between the `rustc::middle::CrateStore` trait (and trait object) and the `rustc_metadata::cstore::CStore`, with an `Rc<CrateStore>` stored in the `Session`. The inner `CStore` can be accessed via the inner `Any` bound, but this is deprecated.

r? @nikomatsakis

tests & rustdoc still broken

this is a [breaking-change] to all plugin authors - sorry

rustdoc still broken

Closes https://github.com/rust-lang/rust/issues/30008

`#[stable]`, `#[unstable]` and `#[rustc_deprecated]` are now guarded by `#[feature(staged_api)]`

r? @brson

Fixes #27092
Fixes #30005

The graph extent mechanism is not good. I have some ideas for a better replacement, but this PR simply removes it. It also stops recursing on statement scopes and processes them using an "on the heap" stack, which fixes #29466.

r? @dotdash

Part of #29428

Part of #29428

`socket_addr` was renamed to `local_addr` in 1.0beta.

See: f798674b

r? @steveklabnik

`socket_addr` was renamed to `local_addr` in 1.0beta.

See: f798674b

This was to support rustpkg but is unused now.