WIP: Support for copying environments with cycles and metatables

[FSMaxB]

This uses breadth first search for copying in order to make it possible to clone tables with reference cycles and metatables.

Fixes #10 and #9

This will fix object oriented interfaces inside the sandbox.

This means that the only remaining use cases for the reference option are:

1. When the table uses too much memory.
2. When it takes too much time to copy the table.