amcheck verification for GiST and GIN

Hello! My name is Grigory Kryachko, I decided to join the efforts with
Andrey Borodin in his working on amcheck.

Here is the patch which I (with Andrey as my advisor) built on the top of
the last patch from this thread: https://commitfest.postgresql.org/25/1800/
.
It adds an ability to verify validity of GIN index. It is not polished
yet, but it works and we wanted to show it to you so you can give us some
feedback, and also let you know about this work if you have any plans of
writing something like that yourselves, so that you do not redo what is
already done.

In the mentioned above thread there was an issue with right type of lock,
we have not addressed it yet. Right now I am primarily interested in
feedback about GIN part.