I'm having a really painful time figuring this out.  I jumped onto
what NoKarma had already begun, with his curl-ffi library, and started
running with it.  I'm trying to emulate the ruby extension layers used
in some different Curl gems (Streamly and Patron, atm).

Curl wants you to pass a method reference, which it will then
callback.  You can see examples used in Streamly and Patron:

Streamly, lines 29-83:
https://github.com/aitrus/streamly/blob/master/ext/streamly.c#L29

Patron, lines 59-75 (Much simpler):
https://github.com/aitrus/patron/blob/master/ext/patron/session_ext.c#L59

Basically, they're allocating a buffer and providing a callback + that
buffer, which is sent to the callback. I've added this callback to
CurlFFI and I have the Proc working, etc... But I cannot seem to get
it to append to the string.

Callback Additions to CurlFFI:
https://github.com/aitrus/curl_ffi/blob/master/lib/bindings.rb#L923
https://github.com/aitrus/curl_ffi/blob/master/lib/bindings.rb#L926

I'll delete 926, as it's not necessary (I think).

Procs that get called:
https://github.com/aitrus/streamly/blob/master/lib/streamly/request.rb#L11

I suppose the last resort can be to call C libraries, allocating /
freeing a string.  But that makes me think I'm doing it wrong.  I
can't imagine that I'm doing anything overly interesting with my code.

Scott