- Refactor lane proxy implementation: it is now a full userdata instead of a table, and its methods are implemented in C instead of Lua. * its metatable is no longer accessible. * writing to the proxy raises an error. * it is no longer possible to overwrite its join() and cancel() methods - when a deep userdata idfunc requests a module to be required, manually check that it is not loaded before requiring it instead of relying on the require function's loop detection feature. - when a module must be required, raise an error if the 'require' function is not found in the target state. - we know Lanes is loaded in the master state, so we don't force it to be required in every lane too when a linda deep userdata is copied.