Switched Node::operator=(const Node&) to *not* force itself to create its node first (since we're just assigning them)

Switched the node_ref to *always* create its data (since now the Node itself doesn't always create itself)

Set the pimpl node in Node to be optional, so we don't create unnecessary guys every time you call Node tmp = foo[value];

Fixed new API node key/value insertion in NodeBuilder (it was using the wrong condition on when it had added a key already)

Implemented std::map decode (and fixed bug in the Node iterator - the reference_type should be just a plain value, since it's created on-the-fly)

Added mutable operator[] for integral types (you can only grow the sequence if you specify the *next* element)