Better __index lookup loop detection

[Xmilia Hermit <xmilia.hermit@...>]

Hi!

Currently loops in the __index lookup are detected by restricting the 
lookup to a depth of 2000.
This can be removed by detecting loops instead of using a depth.
Using two iterators traversing the __index lookup chain. Therby one is 
faster (in this case twice) as the other.
In case there is a loop the faster one will at some point catch up to 
the slower one. If this happens a loop was detected and an error can be 
raised.

The implementation can be found under 
https://github.com/XmiliaH/lua/tree/better-index-loop-detection.

Regards,
Xmilia