detect circularity when constructing an environment

[moodymudskipper]

This will run forever, though it doesn't have to :

env <- new.env() env$e <- env construct(env) We should keep a table of visited environments, and fall back to new.env() when we find circularity. Maybe this should be done for first release since it's a bad look when nothing happens.

[moodymudskipper]

moved since now default behavior doesn't trigger this