- Properly address Variant Ctors
- Show signature if span of trait method without `self` is not available

No longer parse it.
Remove AutoTrait variant from AST and HIR.
Remove backwards compatibility lint.
Remove coherence checks, they make no sense for the new syntax.
Remove from rustdoc.

These no longer work now that Cargo changes the cwd of rustc while it's running.
Instead use an absolute path that's set by rustbuild.

The Generics now contain one Vec of an enum for the generic parameters,
rather than two separate Vec's for lifetime and type parameters.

Additionally, places that previously used Vec<LifetimeDef> now use
Vec<GenericParam> instead.

Renaming MethodRibKind to TraitOrImplItemRibKind and removing its field which was never used. Lifting the HasTypeParameters rib to all trait item kinds and all impl item kinds

This commit adds support for nested groups inside `use` declarations,
such as `use foo::{bar, sub::{baz::Foo, *}};`.

Try to identify the following code in order to provide better
diagnostics, but return the error to bail out early during the parse.

When encountering `pub ident`, attempt to identify the code that comes
afterwards, wether it is a brace block (assume it is a struct), a paren
list followed by a colon (assume struct) or a paren list followed by a
block (assume a fn). Consume those blocks to avoid any further parser
errors and return a `Placeholder` item in order to allow the parser to
continue. In the case of unenclosed blocks, the behavior is the same as
it is currently: no further errors are processed.

On the case of duplicated names caused by an `extern crate` statement
with a rename, don't include the inline suggestion, instead using a span
label with only the text to avoid incorrect rust code output.

When encountering a let binding type error, attempt to parse as
initializer instead. If successful, it is likely just a typo:

```rust
fn main() {
    let x: Vec::with_capacity(10);
}
```

```
error: expected type, found `10`
 --> file.rs:3:31
  |
3 |     let x: Vec::with_capacity(10, 20);
  |         --                    ^^
  |         ||
  |         |help: did you mean assign here?: `=`
  |         while parsing the type for `x`
```