read comments from fields of anonymous structs

[arvidfm]

This PR adds support for reading documentation comments from fields of anonymous structs. The code keeps track of what fields it has traversed, and builds up a breadcrumb that includes every parent field.

Consider the following fields:

type A {
    Val1 int
    Val2 struct {
        StructVal int
    }
    Val3 []struct {
        SliceVal int
    }
    Val4 map[struct {
        MapKeyVal int
    }]struct {
        MapValueVal int
    }
}

These fields will, respectively, have the following breadcrumbs, used (along with the fully qualified name of the package) as the key in the comments map:

• A.Val1
• A.Val2.StructVal
• A.Val3.[].SliceVal
• A.Val4.[key].MapKeyVal
• A.Val4.[value].MapValueVal

Note that any time the code encounters a non-anonymous type, it resets the breadcrumb to the type's fully qualified name.

Tested with a decently large codebase.

[arvidfm]

@samlown I've added tests now

[arvidfm]

@samlown I cleaned up the breadcrumb tracking a bit to make the code more readable, and I also added a fix so that comments of generic classes are fetched correctly. Let me know if there's anything else I could do to help upstream this, since the lack of support for anonymous structs is forcing us to use a fork for our documentation at the moment.

Instead of explicitly passing the breadcrumb/path as a parameter everywhere, I also toyed with the idea of a pattern like:

r.withField(f.Name, func() {
	// f.Name is at the top of the breadcrumb stack here
	property = r.refOrReflectTypeToSchema(definitions, f.Type)
})
// f.Name has been popped from the stack here

This would greatly reduce the number of places where you need to interact with the breadcrumbs, but it's also a bit volatile since it's easier to forget to include it. Let me know if it looks a bit more palatable to you, though.

Alternatively, perhaps the cleanest solution would be to add thin wrappers around reflect.Type/reflect.StructField that automatically track how types are accessed when you call Elem()/Field()/etc and provide FullyQualifiedName() methods that works for anonymous structs too.