46: Add impl block support to #[project] attribute r=taiki-e a=taiki-e

This adds a feature to convert an annotated impl to a projected type's impl.

Example:

```rust
#[project]
impl<T, U> Foo<T, U> {
    fn baz(self) {
        let Self { future, field } = self;
    }
}

// convert to:
impl<'_pin, T, U> __FooProjection<'_pin, T, U> {
    fn baz(self) {
        let Self { future, field } = self;
    }
}
```

Closes #43

cc @Nemo157



48: Assert lack of Drop impl without implementing Drop r=taiki-e a=Aaron1011

Previously, we would provide an empty Drop impl for types that did not
provide a pinned_drop method, in order to assert that the user was not
providing their own Drop impl.

However, the compiler places certain restrictions on types that
implement Drop. For example, you cannot move out of the fields of such
types, as that would cause the Drop impl to observe the fields in an
uninitialized state.

This commit changes to a purely trait-based approach, as used in
the assert-impl crate by @upsuper . This prevents the creation of
unnecessary Drop impls, while still ensuring that the user cannot
provide their own Drop impl without going through pinned_drop

Co-authored-by: Taiki Endo <te316e89@gmail.com>
Co-authored-by: Aaron Hill <aa1ronham@gmail.com>

109: Release 0.4.0 r=taiki-e a=taiki-e

cc #21

### Changes since the latest 0.3 release:

* **Pin projection has become a safe operation.** In the absence of other unsafe code that you write, it is impossible to cause undefined behavior. (#18)

* `#[unsafe_project]` attribute has been replaced with `#[pin_project]` attribute. (#18, #33)

* The `Unpin` argument has been removed - an `Unpin` impl is now generated by default. (#18)

* Drop impls must be specified with `#[pinned_drop]` instead of via a normal `Drop` impl. (#18, #33, #86)

* `Unpin` impls must be specified with an impl of `UnsafeUnpin`, instead of implementing the normal `Unpin` trait. (#18)

* `#[pin_project]` attribute now determines the visibility of the projection type/method is based on the original type. (#96)

* `#[pin_project]` can now be used for public type with private field types. (#53)

* `#[pin_project]` can now interoperate with `#[cfg()]`. (#77)

* Added `project_ref` method to `#[pin_project]` types. (#93)

* Added `#[project_ref]` attribute. (#93)

* Removed "project_attr" feature and always enable `#[project]` attribute. (#94)

* `#[project]` attribute can now be used for `impl` blocks. (#46)

* `#[project]` attribute can now be used for `use` statements. (#85)

* `#[project]` attribute now supports `match` expressions at the position of the initializer expression of `let` expressions. (#51)

### Changes since the 0.4.0-beta.1 release:

* Fixed an issue that caused an error when using `#[pin_project(UnsafeUnpin)]` and not providing a manual `UnsafeUnpin` implementation on a type with no generics or lifetime. (#107)


Co-authored-by: Taiki Endo <te316e89@gmail.com>