Add #[allow(unsafe_code)] to projection macros

[taiki-e]

I feel the current behavior of lint is a little odd, but I'd like to prevent unsafe macros from being used on the crate that forbids unsafe_code.

This may break some crate, but pin-utils is still alpha, so should be acceptable.

Closes #2

r? @cramertj
cc @Nemo157

[Nemo157]

Opened rust-lang/rustup#2065 about the CI failure, it's fallout from the last rustup release.

Since Pin is stable now it might be a good idea to move the clippy job over to using the stable channel so it will be guaranteed to exist.

[Nemo157]

new rustup released which should fix this, so rerunning this build should work (but I'll probably still look at updating to use stable later).

[Nemo157]

Still broken because the cache has stored the broken state... if someone has access to clear the cache it should fix it.