API for storing and retrieving "private" data

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

src/luerl.erl

@@ -46,6 +46,9 @@
 %% Helping with storing VM state
 -export([externalize/1,internalize/1]).
 
+%% Storing and retrieving private data
+-export([put_private/3,get_private/2]).
+
 %% init() -> State.
 
 init() ->
@@ -506,3 +509,10 @@ externalize(S) ->
 
 internalize(S) ->
     luerl_lib_math:internalize(S).
+
+put_private(Key, Value, S) ->
+    Private = maps:put(Key, Value, S#luerl.private),
+    S#luerl{private=Private}.
+
+get_private(Key, S) ->
+    maps:get(Key, S#luerl.private).

src/luerl.hrl

@@ -34,7 +34,8 @@
                 rand,                           %Random state
                 tag,                            %Unique tag
                 trace_func=none,                %Trace function
-                trace_data                      %Trace data
+                trace_data,                     %Trace data
+                private=#{}
                }).
 
 %% Table structure.