getCache was still getting serializable issues

[ThePrimeagen]

So instead of doing

x[0] !== prefix && x !== '$size';

we switched it up with a more verbose check.

module.exports = function isInternalKey(x) {
    return x === __parent ||
        x === __key ||
        x === __version ||
        x === __prev ||
        x === __next ||
        x === "$size";
};

#601

[trxcllnt]

@michaelbpaulson @jhusain @sdesai this check used to catch all our "internal" keys (the ones with the prefix), but now internal keys (like back-references) are leaking into the result of getCache and confusing people :(

[sdesai]

I'm re-establishing this (just with charAt(0) instead of the index lookup, just to be safe), in the 0.x branch as we speak (we just hit it internally too - the refN, ref-index, etc. keys ended up in the cache).

Was just trying to collect context from @jhusain @michaelbpaulson on why we ended up going with the new impl in this PR, in 0.x, and adding unit tests, and will be ready for new PR tomorrow.