rust: pin-init: rewrite `#[pin_data]` using `syn`

rust: pin-init: rewrite `#[pin_data]` using `syn`

Rewrite the attribute macro `#[pin_data]` using `syn`. No functional
changes intended aside from improved error messages on syntactic and
semantical errors. For example if one forgets a comma at the end of a
field:

    #[pin_data]
    struct Foo {
        a: Box<Foo>
        b: Box<Foo>
    }

The declarative macro reports the following errors:

    error: expected `,`, or `}`, found `b`
     --> tests/ui/compile-fail/pin_data/missing_comma.rs:5:16
      |
    5 |     a: Box<Foo>
      |                ^ help: try adding a comma: `,`

    error: recursion limit reached while expanding `$crate::__pin_data!`
     --> tests/ui/compile-fail/pin_data/missing_comma.rs:3:1
      |
    3 | #[pin_data]
      | ^^^^^^^^^^^
      |
      = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`$CRATE`)
      = note: this error originates in the macro `$crate::__pin_data` which comes from the expansion of the attribute macro `pin_data` (in Nightly builds, run with -Z macro-backtrace for more info)

The new `syn` version reports:

    error: expected `,`, or `}`, found `b`
     --> tests/ui/compile-fail/pin_data/missing_comma.rs:5:16
      |
    5 |     a: Box<Foo>
      |                ^ help: try adding a comma: `,`

    error: expected `,`
     --> tests/ui/compile-fail/pin_data/missing_comma.rs:6:5
      |
    6 |     b: Box<Foo>
      |     ^

Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>