rust: pin-init: rewrite the initializer macros using syn

Rewrite the initializer macros `[pin_]init!` using `syn`. No functional
changes intended aside from improved error messages on syntactic and
semantical errors. For example if one forgets to use `<-` with an
initializer (and instead uses `:`):

    impl Bar {
        fn new() -> impl PinInit<Self> { ... }
    }

    impl Foo {
        fn new() -> impl PinInit<Self> {
            pin_init!(Self { bar: Bar::new() })
        }
    }

Then the declarative macro would report:

    error[E0308]: mismatched types
      --> tests/ui/compile-fail/init/colon_instead_of_arrow.rs:21:9
       |
    14 |     fn new() -> impl PinInit<Self> {
       |                 ------------------ the found opaque type
    ...
    21 |         pin_init!(Self { bar: Bar::new() })
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       |         |
       |         expected `Bar`, found opaque type
       |         arguments to this function are incorrect
       |
       = note:   expected struct `Bar`
               found opaque type `impl pin_init::PinInit<Bar>`
    note: function defined here
      --> $RUST/core/src/ptr/mod.rs
       |
       | pub const unsafe fn write<T>(dst: *mut T, src: T) {
       |                     ^^^^^
       = note: this error originates in the macro `$crate::__init_internal` which comes from the expansion of the macro `pin_init` (in Nightly builds, run with -Z macro-backtrace for more info)

And the new error is:

    error[E0308]: mismatched types
      --> tests/ui/compile-fail/init/colon_instead_of_arrow.rs:21:31
       |
    14 |     fn new() -> impl PinInit<Self> {
       |                 ------------------ the found opaque type
    ...
    21 |         pin_init!(Self { bar: Bar::new() })
       |                          ---  ^^^^^^^^^^ expected `Bar`, found opaque type
       |                          |
       |                          arguments to this function are incorrect
       |
       = note:   expected struct `Bar`
               found opaque type `impl pin_init::PinInit<Bar>`
    note: function defined here
      --> $RUST/core/src/ptr/mod.rs
       |
       | pub const unsafe fn write<T>(dst: *mut T, src: T) {
       |                     ^^^^^

Importantly, this error gives much more accurate span locations,
pointing to the offending field, rather than the entire macro
invocation.

Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin