Rewrite macro 'with-curl-easy'

[fraya]

Use a variable to initialize the curl handler, instead of creating the class inside the macro. This allows to subclass <curl-easy>.

with-curl-easy(curl = make(<curl-easy>))
  curl.curl-url := "http://example.com";
  curl.curl-verbose := #f;
  curl-easy-perform(curl);
end;

Now is possible to pass options to the macro. It makes less verbose the code.

with-curl-easy(curl = make(<curl-easy>),
               url = "http://example.com",
               verbose = #f)
  curl-easy-perform(curl)
end;

Closes #8