Add serialization for std::optional

[sdebionne]

Fixes #114 together with #148.

[robertramey]

I don't see any tests for this. That every "built-in" type which includes serialization code must be tested. That is must have a test program written for it. It's not hard - look at the rest of them. But it is a requirement. Looking forward to this.

[sdebionne]

Speaking of the tests, the CI is broken since Dec 16, 2020. The std::variant has tests but was never merged. So why should I bother?

[Ulysses1337]

What I hope to do one day is replace all my Boost optionals with std. Can we name the bool field "initialized" for compatibility?

[robertramey]

I merged this and pulled into my local repo. I was considering writing a test for it. Upon closer examination I found there is an implementation for boost::optional serialization which is almost identical to your implementation of serialization for std::optional. So it seems to me that we should include serialization for both boost::optional and std::option in the same file. And the test for optional should be extended to include std::optional as well. This will diminish the code which needs to be maintained in the future.

As an aside, I think you should have credited the original author of the serialization for boost::optional in your header.

[sdebionne]

Of course it is almost identical as boost and std optionals have the same interfaces.

So it seems to me that we should include serialization for both boost::optional and std::option in the same file

Why force people that use std::optional to depend on boost::optional via this shared header?

the test for optional should be extended to include std::optional as well

Agreed but given that std::variant did not get merged, I did not want to spend time on adding the test here.

As an aside, I think you should have credited the original author of the serialization for boost::optional in your header.

OK. I would create a PR for that but since it has been reverted in #299, I won't.

[robertramey]

I've incorporated your working boost/serialization/optional and boost/serialization/variant. I think you'll find this satisfactory. Feel free to review it and ask me about it.