Support Option<Option<T>> field types

[sphynx]

Closes #188

I used sub_type function in Attrs module, so I moved it to there and exported so that it can be reused in lib.rs, I am not sure this is an optimal solution, please let me know if you want to change that (for example to have a separate module for types manipulations or anything else).

Another thing that I wanted to add to tests: is to make sure that Option<Option<T>> cannot be used for positional arguments. In this case I would expect a macro code-generation to fail, but I'm not sure how to make a test which expects particular macro failure. For now, I just checked that it works manually.

Also please note that I am using max_values=1 as well as min_values=0 to limit number of values to 1 in the generated clap code. However, handling of max_option in clap apparently has a bug: it tries to consume one more argument than it should during the parsing.

I fixed that bug and prepared a PR here: clap-rs/clap#1481
However, before that bug is fixed, theoretically we can have problems if, say, Option<Option<T>> long field with a value is immediately followed by a subcommand, like this:

test --field 1 add -p 2

then clap (without my fix) would continue consuming arguments for --field even when max_values=1 is set, so it will consume subcommand add as well and parsing will fail on seeing -p. But this is not directly related to Option<Option<T>> feature, just something to keep mind since it's related to max_values.

Please let me know if you have any suggestions about the code.

[TeXitoi]

you need to rustfmt please.

[TeXitoi]

Great work! Only a minor stylistic remark.

[TeXitoi]

No problem for the sub_type move.

There is no (yet) compile failure tests, so I don't ask to do this. But I'll appreciate a dedicated PR adding this new infrastructure ;-)

Noted for the clap problem, can be useful if a related bug report rises.

[TeXitoi]

Also can you update the CHANGELOG.md? Thanks.

[TeXitoi]

I forgot a last thing: this feature must be documented.

[sphynx]

I've fixed everything you mentioned.

Not sure about CHANGELOG format: should it be under a new version header? For now I've just added a new item on the very top without the version header. I can bump the package version as well (but perhaps you'd like to merge other pull requests first).

Thanks for your feedback!

[TeXitoi]

Thanks!

[TeXitoi]

v0.2.16 published