Option to generate pointers to fields on struct pointers

[dcharkes]

After we address dart-lang/sdk#41237.

We can add an option to generate extension methods for Pointers to fields of Pointer<MyStruct>:

If we settle on an API like this for getting the offsets:

class MyStruct extends Struct {
  @Int32()
  external int foo;
  @Int64()
  external int bar;

  @OffsetOf(#foo)
  external const int offsetOfFoo;
  @OffsetOf(#bar)
  external const int offsetOfBar;
}

Then we could generate extension methods such as:

extension MyStructFields on Pointer<MyStruct> {
  Pointer<Int32> foo => Pointer.fromAddress(address + MyStruct.offsetOfFoo);

  Pointer<Int64> bar => Pointer.fromAddress(address + MyStruct.offsetOfBar);
}

Which would be handy for cases like dart-archive/ffi#117.