Add support for structures and enumerations.

[KlimMixer]

Fixes: #266

[frangio]

Thanks @KlimMixer!

[KlimMixer]

@frangio Thank you. Updated, and fixed all problems.

[frangio]

I was wrong about documentation, these nodes don't have documentation in the AST. I also decided to simplify things a little, enum and struct members don't need to be Linkable. Please take a look and let me know if you have any feedback.

[KlimMixer]

Thank you for the simplification and nice realization of differentiation between functions events modifiers and other contract items.
And next, I will write tests and push for the review.

[frangio]

For testing you should make sure there are structs and enums in one of the fixtures like fixtures/basic for example.

[frangio]

Can you create a single new fixture with both struct and enum? And give it a custom template (see how custom does it) that specifically tests struct and enum and its members.

[frangio]

Looks good. Thank oyu @KlimMixer!