Add support for self referencing schema (#375)

Previously field with type `Box<Self>` within a `ToSchema` struct or
enum did not create correct OpenAPI schema but mistakenly created a
reference to unknown type `#/components/schemas/Self`.

This commit fixes this issue and now correct type will be created when
`Self` keyword is being used withing `ToSchema` structs or enums. E.g.
now `struct Item(Box<Self>);` will create a reference like
`#/components/schemas/Item`.

This commit also makes a breaking change to `ToSchema` trait where
previously it was not possible to add a Reference directly to OpenAPI
components schemas map even this is allowed by OpenAPI spec. This is
commit will change the `ToSchema` return type to `RefOr<Schema>` to
allow returning references directly.