Add support for #[project(!Unpin)]

[matheus-consoli]

This PR adds support to making types !Unpin.

Ex.:

pin_project! {
    #[project(!Unpin)]
    struct Foo<A, B> {
       #[pin]
       a: A,
       b: B,
    }
}

We also hide the documentation of the implementation that makes the type !Unpin, so it should not show in the rustdoc sidebar.

closes: #75

[taiki-e]

LGTM. Thanks!

[taiki-e]

Published in 0.2.11.