Implement forwarding visitors for Option<V>

[apasel422]

Fixes #183

[martinthomson]

Oh, I like this better than mine, I think. Please take a look at the places where I added Option returns and maybe only implement for those. I don't think that it is necessary to have this for an ItemListVisitor, for example.

[apasel422]

Oh, I like this better than mine, I think. Please take a look at the places where I added Option returns and maybe only implement for those. I don't think that it is necessary to have this for an ItemListVisitor, for example.

IIUC all of the ones I've added so far are necessary. Please take a look and let me know what you think.

[martinthomson]

Would you mind updating the tests that I wrote to include Some/None as appropriate. I think that you can cherry-pick from #185 for the test changes.

[valenting]

This is great, thanks!

Going forward I wonder if we should also try implementing some helpers for the most common uses for parsing. For example, when parsing a dictionary, get the value of an item named x that is expected to be of type string, and (optionally) its parameter y which should be an integer.

[apasel422]

Going forward I wonder if we should also try implementing some helpers for the most common uses for parsing. For example, when parsing a dictionary, get the value of an item named x that is expected to be of type string, and (optionally) its parameter y which should be an integer.

I'd be happy to consider that once we get some more feedback, though it would also be worth investigating a serde implementation, at least for deserialization. For now I think we should bump to 0.12.0 and do a release.