[ffigen] Block support (#333)

* WIP: Blocks

* Move ObjCBuiltInFunctions to its own file and its singleton to data.dart

* Block parsing

* Add a test

* More WIP on blocks

* WIP test

* Test is working (with a few hacks)

* Add pointer getter

* Default values for exceptional return

* Fix tests

* Fix analysis errors

* Update test expectations

* Merge cruft

* Merge