rust: pin-init: internal: use marker on drop guard type for pinned fields

rust: pin-init: internal: use marker on drop guard type for pinned fields

Instead of projecting the created reference, simply create drop guards with
different marker types and have the `let_binding()` method of guards of
different marker produce different type instead.

This allows more flexible lifetime as this is now controlled by the guard.
This will be needed when implementing self-referential fields.

Signed-off-by: Gary Guo <gary@garyguo.net>