In cdcd82679c, we rewrote the Lua
conversion function to update the "Java -> Lua" mapping after
conversion, rather than part way through.
This made the code a little cleaner (as we only updated the mapping in
one place), but is entirely incorrect — we need to store the object
first, in order to correctly handle recursive objects — otherwise we'll
just recurse infinitely (or until we overflow).
This partially reverts the above commit, while preserving the new
behaviour for singleton collections.
Fixes#1955.
0069591af9