internal: pin_data: support tuple struct projections

Add plumbing for tuple struct support to `#[pin_data]` by generating
tuple struct projections and tuple-aware pin-data metadata.

Keep the projection API native to Rust tuple structs, so projected
fields are accessed as `.0`, `.1`, etc. instead of introducing
synthetic public field names.

Signed-off-by: Mohamad Alsadhan <mo@sdhn.cc>

Author: mqqz

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Support tuple structs in `#[pin_data]`, including tuple struct projections.
 - `[pin_]init_scope` functions to run arbitrary code inside of an initializer.
 - `&'static mut MaybeUninit<T>` now implements `InPlaceWrite`. This enables users to use external
   allocation mechanisms such as `static_cell`.