[ffi] Convert allocators to use @Native

[dcharkes]

This would make them compatible with dart2wasm if that exports the right symbols:

@ffi.Native<ffi.Pointer<ffi.Void> Function(ffi.Int)>(symbol: 'malloc')
external ffi.Pointer<ffi.Void> _wasmAllocate(
  int size,
);

@ffi.Native<ffi.Void Function(ffi.Pointer<ffi.Void>)>(symbol: 'free')
external void _wasmDeallocate(
  ffi.Pointer<ffi.Void> ptr,
);

class _WasmAllocator implements ffi.Allocator {
  @override
  ffi.Pointer<T> allocate<T extends ffi.NativeType>(
    int byteCount, {
    int? alignment,
  }) {
    return _wasmAllocate(byteCount).cast<T>();
  }

  @override
  void free(ffi.Pointer<ffi.NativeType> pointer) {
    _wasmDeallocate(pointer.cast());
  }
}

final calloc = _WasmAllocator();

Thanks @TheComputerM!

[sam-p-at-pieces]

@dcharkes Not sure exactly how this works but I'm running into a compilation issue when build for web with flutter build web --wasm:

Target dart2wasm failed: ProcessException: Process exited abnormally with exit code 64:
../../../.pub-cache/hosted/pub.dev/ffi-2.1.3/lib/src/allocation.dart:24:22: Error: Argument to 'Native.addressOf' must be annotated with @Native.
    Native.addressOf(posixFree);

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