This commit increases the convenience of using the MsgPackDocument API,
especially when creating a document for writing out.
It adds direct assignment of bool, integer and string types to a
DocNode, as long as that DocNode is already inside a document, e.g. the
result of a map lookup. It also adds map lookup given an integer type
(it already had that for string).
So, to assign a string to a map element whose key is an int, you can
MyMap = "towel";
MyMap[MyMap.getDocument()->getNode(42)] = MyMap.getDocument()->getNode("towel");