Improved compatibility with old coroutines

[ghost]

It checks if debug or getfenv are avaliable, and uses them to create a new "coroutine" table inside coroutines upon coroutine creation. Thus, coroutines created inside coroutines won't yield to where "coroutine" table was created. Also added 'optional' depth limit (not really optional, but defaults to math.huge, which is infinite when Lua is compiled with floating point, which is how it is on most popular hardware.)

[saucisson]

Thanks, can you please explain me the goal of your patch, and what it fixes?

[saucisson]

Sorry, i have some urgent work. I will come back to discuss your patches in a few days.