refactor(copy): add Writer object

refactor(copy): add Writer object

This separation of responsibility will allow to:

- create writers that e.g. produce a file rather than writing to a
connection;

- create writers that write directly instead of using a queue (see
https://github.com/psycopg/psycopg/discussions/111 for an example of users
unhappy about that. I still don't see their nervousness justified, but
I'm ok to provide them an escape hatch if it's easy to implement).