The main problem in using the nested coroutines library is that,
usually, Lua scripts use the global `coroutine` variable, instead of
creating a local copy of it. So we can't just replace `_G.coroutine`
with a nestable one before requiring the module.
Search for instance the use of `coroutine` in:
* https://github.com/keplerproject/copas/blob/master/src/copas.lua
* https://github.com/nrk/redis-lua/blob/version-2.0/src/redis.lua
If modules used a local `coroutine` variable, we could at least
monkey-patch libraries as below:
local coromake = require "coroutine.make"
_G.coroutine = coromake ()
local copas = require "copas"
_G.coroutine = coromake ()
local redis = require "redis"
_G.coroutine = coromake ()
