rust: macros: convert `#[vtable]` macro to use `syn`

rust: macros: convert `#[vtable]` macro to use `syn`

`#[vtable]` is converted to use syn. This is more robust than the
previous heuristic-based searching of defined methods and functions.

When doing so, the trait and impl are split into two code paths as the
types are distinct when parsed by `syn`.

Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20260112170919.1888584-4-gary@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>