rust: macros: rewrite #[pin_data] using syn

Now that `syn` and `quote` are available in the Kernel use them for the
`#[pin_data]` proc-macro attribute instead of the current hybrid
proc-macro/declarative approach.

Proc-macros written using `syn` are a lot more readable and thus easier
to maintain than convoluted declarative macros. An additional advantage
of `syn` is the improved error reporting, here is an example:

    #[pin_data]
    struct Foo {
        x
    }

Prior to this patch, the error reads:

    error: expected `:`, found `}`
      --> samples/rust/rust_minimal.rs:43:1
       |
    41 | struct Foo {
       |        --- while parsing this struct
    42 |     x
       |      - expected `:`
    43 | }
       | ^ unexpected token

    error: recursion limit reached while expanding `$crate::__pin_data!`
      --> samples/rust/rust_minimal.rs:40:1
       |
    40 | #[pin_data]
       | ^^^^^^^^^^^
       |
       = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`rust_minimal`)
       = note: this error originates in the macro `$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)

    error: aborting due to 2 previous errors

After this patch, the error reads:

    error: expected `:`, found `}`
      --> samples/rust/rust_minimal.rs:43:1
       |
    41 | struct Foo {
       |        --- while parsing this struct
    42 |     x
       |      - expected `:`
    43 | }
       | ^ unexpected token

    error: aborting due to 1 previous error

Signed-off-by: Benno Lossin <[email protected]>

Author: BennoLossin