Add comments extraction support

[olbat]

This change introduces a new option to import C doctrings comments at parsing time. It also introduce a new "import_docstrings" parameter to import C doctrings as Crystal comments when generating a lib.

This parameter can take two values: "brief" to extract the brief part of doctrings or "full" to extract the whole docstring ^1,2.

A bunch of changes have been made to the core lib to make it possible:

• an Option enum has been added to CrystalLib::Parser
• an @options attribute has been added to CrystalLib::Parser
• a @doc attribute has been added to CrystalLib::AST nodes

[olbat]

Note: tests are working fine but I didn't manage to get it working with a real example yet, I'm waiting for your feedbacks on the idea of this PR before investigating further.

Also, this change depends on crystal-lang/clang.cr#7.

[olbat]

(I think that the failed CI job should just be restarted for it to pass)

[zph]

I was able to try out this branch and it was able to extract some of the comments from my header file 👍. It doesn't get all of them but that might be due to multiple valid comment formats in the example I tried.

[olbat]

@zph so far I could only parse Doxygen comments but I think there might be an option somewhere to make it possible to parse every type of comments because there are some tests for that in LLVM sources.

[olbat]

Is it ready to be merged, are still some stuffs to be fixed/improved?