We go very explicit here and assume that all Luerl functions require
their arugments to be explicitly encoded and return values
decoded. This is still a test version.

Also commented out some test output.

So luerl_new has now become the luerl. For backwards compatibility
save the old luerl unchanged as luerl_old. The documentation and test
suites have updated as well. A major change in luerl is the
encoding/decoding of functions which are assumed to use encoded
arguments and return values.

These just call the erlang module functions moving the state to the
first argument.

Fix tests on develop-encode

Adds an API that allows a consumer of Luerl to store private data that
can be retrieved from functions called within Lua. This is useful in
applications that may need to retrieve secrets from within Luerl, but
don't want Lua scripts to be able to access these values.

I've chatted with a number of users of Luerl, including Sam Aaron, who
have had the need to expose secrets into their Luerl programs. Some have
achieved this by using closures to close over private data and expose
those functions to Luerl. I have used the process dictionary to acheive
the same thing.

If Luerl had an API to store and retrieve private data, as proposed by
this PR, then both techniques would be unnecessary.

Provide a `update_private/2` function instead that takes a function that
can modify the private map

- [ ] Get a nod from Robert
- [ ] Implement in other "Elixir" API
- [ ] Decide if we need to implement for "old"
- [ ] Tests
- [ ] Documentation

API for storing and retrieving "private" data

Also update luerl_sandbox.erl.