Support nested structs

[jpnurmi]

Fixes dart-lang/native#541

[mannprerak2]

We can also target dart-lang/native#540 along with this.

And It is better to put these behind a flag in config for users to enable this, since this will only work on dart master branch as of now.

I think this should be held off until we merge #122 .

[dcharkes]

Super cool stuff! 👍

I second @mannprerak2, we should not merge this into the master branch until nested structs also becomes available in Dart stable. If we merge it into master we can no longer make any changes to package:ffigen and release them for Dart stable in the mean time.
(We could make a new branch dart-master which we build against the master branch of Dart as you did in this PR in test-package.yml. And release a 1.3.0-nested-structs.0 version. However, you can also just use a git dependency to depend on your own branch for the time being. Any preference @jpnurmi ?)

@mannprerak2 do you want to do the reviewing for this PR?

[jpnurmi]

Sure, I was planning to get back to this after #122 has been merged. This can be kept in an experimental branch or made configurable as @mannprerak2 suggested. There's no rush from my side to have this upstream, I got the files already generated. :)

[mannprerak2]

@mannprerak2 do you want to do the reviewing for this PR?

Sure.

[mannprerak2]

@jpnurmi I now feel maybe it's better if we do create an experimental branch for this (dart-master as Daco suggested). Otherwise, we'd need to add and then later remove those configs, modify tests to run according to certain configurations (lots of stuff which would need to be reverted later on).

Also, we can target dart-lang/native#540 as a separate PR, since it needs to know if a struct had been properly generated to be able to pass it by value correctly.

[mannprerak2]

@dcharkes I think you would need to create a dart-master branch so that @jpnurmi can make a PR on it.

[dcharkes]

Done: https://github.com/dart-lang/ffigen/tree/dart-master