Refactor `GOBL::Schema::Object` embed and extract method usage into `GOBL::Envelope`

* After the removal of "GOBL::Document", we want to reduce the protagonism of the `GOBL::Schema::Object` that replaces it into an implementation detail instead of a main point of contact with the library.
* `GOBL::Envelope` should be aligned with the Go library's "insert" and "extract" methods that deal with the schema handling behind the scenes.

See also #55 