This change drops support for "protecting" raw Lua functions.
There are two main reasons for this change:
* More modern versions of PUC Rio Lua don't have `setfenv`. It is
possible to get around this by using the debug library, but that
library is not available in all environments.
* Solutions based on `load` (which only allow string inputs) are
objectively better since they give the user more control. For
instance, you can deactivate support for binary code selectively.
As a result, we are using the `load`-based sandbox in all versions of
Lua that supports it, using `setfenv`-based sandboxing only when nothing
else is available (PUC Rio 5.1).
We are also explicitly raising an error if `options.mode` is passed but
we are using `setfenv`. This is to prevent users from believing they are
protected against binary code, when in fact they are not.
