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>