Offer native struct constructors

[Levi-Lesches]

When working with C structs, I find I have to have a few lines dedicated to allocating a struct pointer and initializing the fields.

// C API
typedef struct MyStruct {
  int x;
  float y;
  MyEnum z;
} MyStruct;

// ignore_for_file: unused_element
// AUTO GENERATED FILE, DO NOT EDIT.
//
// Generated by `package:ffigen`.
// ignore_for_file: type=lint, unused_import
import 'dart:ffi' as ffi;

final class MyStruct extends ffi.Struct {
  @ffi.Int()
  external int x;

  @ffi.Float()
  external double y;
}

// Application code
final structPtr = arena<MyStruct>()
  ..ref.x = 1;
  ..ref.y = 2.3;

What if ffigen could generate a constructor instead?

// Generated as part of MyStruct
static Pointer<MyStruct> allocate(
  Allocator allocator, {
  required int x,
  required double y,
}) => allocator<MyStruct>()
  ..ref.x = x
  ..ref.y = y;

And then we can instead do

final structPtr = MyStruct.allocate(x: 1, y: 2.3, allocator: arena);

This would be helpful for structs with lots of fields.