liluat.private.clone_table: added support for cloning cyclical tables…

[jleija]

… (and their metatables). Fixes stack overflow error due to recursion.

[jleija]

Hi,

I'm trying to use your library for my work-in-progress library: https://github.com/jleija/stencil :)
Sometimes I have tables with values referencing back to higher-level tables (cyclical), and liluat fails with a stack overflow. I found a deepcopy function that does the trick for cyclical tables and adapted it into your code. I hope the solution is in line with your work.
Let me know if you want me to modify the code to match your conventions/standards.