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>

The `InitData` and `PinData` traits do not need to exist, the inference
helpers could be inherent methods instead.

There is no risk for calling the wrong methods even when user defines it,
as inherent methods take priority over trait methods.

With this change, it unlocks the possibility of attaching additional bounds
to the method per type, which is not possible for trait methods.

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

Instead of projecting using pointer to a field project the full slot. This
further shifts the code generation from the initializer site to the struct
definition site, which means less code is generated overall.

It also makes the safety comment easier to justify, as now the projection
is done by the `#[pin_data]` macro which has full visibility of pinnedness
of fields.

The field alignment could also be checked on the `#[pin_data]` side;
however, since `init!()` macro works for other type of structs, we cannot
remove the alignment check from `init!`/`pin_init!` side anyway, so I opted
to still keep the alignment check in init.rs.

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