Replace coroutines with iterators

[hishamhm]

Let's avoid using coroutines in Penlight for iterators, and use closures instead. Coroutines in Lua do not compose well at a library level, so they are better left to the application who controls the main loop to own and manage. (An exception to this rule of thumb is libraries which are the main loop, such as Copas.)

For more background on this general issue of coroutine composability, see for example:

• https://github.com/mascarenhas/taggedcoro
• https://github.com/saucisson/lua-coronest

This PR also drop OpenResty hacks from Penlight.

[alerque]

Thanks for this. I actually banged my head against this just last night and went to bed without figuring out what was going screwy. In light of this I poked at the same problem today and this solves it. I'm still kind of foggy on why, but my app using coroutines does seem related.

One question though: might it be useful to keep the OpenResty compatibility shim: not for use in Penlight itself but to provide to apps?

[hishamhm]

No, because at Kong we've reached the conclusion that these wrappers do not work correctly. These OpenResty functions are fundamentally incompatible, it's a waste of time to try to coerce them into precise Lua semantics.

(And in my personal opinion, beyond the scope of Penlight anyway; why should OpenResty get special treatment as opposed to all the other applications and games that customize a Lua VM by making it slightly incompatible with the stock language in one way or another...)

[alerque]

I'll buy that, thanks.

[Tieske]

Thx @hishamhm !