Treat the private store as a gc root

[Taure]

luerl:put_private/3 is documented as putting "a private Value under Key
that is not exposed to the runtime". luerl_heap:gc/1's root set is
[Meta..., G | Stk] plus the call stack (luerl_heap.erl:601-602) and does
not include the private store.

So a reference kept there is unreachable from the collector's point of view.
The collector frees the table underneath it, the reference survives as an
opaque term, and the embedder's next decode/2 fails on a reclaimed index:

St0 = luerl:init(),
{Ref, St1} = luerl:encode(#{<<"keep">> => <<"me">>}, St0),
St2 = luerl:put_private(mine, Ref, St1),
St3 = luerl:gc(St2),
luerl:decode(luerl:get_private(mine, St3), St3).
%% ** exception error: {badkey,14}

The failure is delayed and does not point at the collection that caused it,
and the API is exactly the one that invites keeping a reference there.

Adding the store to the root set is one line. Included is a regression test,
which fails on develop with {badkey,14}.

Verification

rebar3 eunit 31/0 (30 existing plus the new test) and rebar3 ct 11/11.

Found while investigating unbounded Luerl memory growth in a game backend.
The workaround without this is to anchor the value in a global for the
duration of the collection and remove it straight after, which is visible to
script code in between.