Update flex-array implementation to work with Rust 1.80

[dtolnay]

The signature of core::ptr::from_raw_parts is different between Rust 1.79 and Rust 1.80, causing an ambiguity in the code previously produced by bindgen.

• https://doc.rust-lang.org/1.79.0/core/ptr/fn.from_raw_parts.html
• https://doc.rust-lang.org/1.80.0/core/ptr/fn.from_raw_parts.html

- pub fn from_raw_parts<T: ?Sized>(
+ pub const fn from_raw_parts<T: ?Sized>(
-     data_pointer: *const (),
+     data_pointer: *const impl Thin,
      metadata: <T as Pointee>::Metadata,
  ) -> *const T

error[E0283]: type annotations needed
   --> lib.rs:317:62
    |
317 |             let p: *const Self = ::core::ptr::from_raw_parts(::core::ptr::null(), len);
    |                                  --------------------------- ^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `null`
    |                                  |
    |                                  required by a bound introduced by this call
    |
    = note: cannot satisfy `_: Thin`
help: consider specifying the generic argument
    |
317 |             let p: *const Self = ::core::ptr::from_raw_parts(::core::ptr::null::<T>(), len);
    |                                                                               +++++