Support for nested structs

[mariotoffia]

E.g. this setup will not extract the fields and documentation:

type Baah struct {
  Foo struct {
    // Baz is a property
    Baz string
  }

The Decl for Baah is now

Foo struct {
  // Baz is a property
  Baz string
}

and that is not correct. It SHOULD be

Baah.Foo.Baz Fax is a property

and the Foo Struct should be

type Baah struct {
  Foo struct {
   Baz string
  }

This requires more job when parsing the structs...