Make bsoncxx value types capable of owning their own memory

The bsoncxx value types are all views into a buffer of memory managed by an owning bsoncxx document.  However, the Client Side Encryption project requires that BSON values be handling and passed around independently of documents; specifically, this supports explicit encrypt() and decrypt() methods, which operate on a per-BSON-value level.  Before CXX-1687 can be completed bsoncxx value types must be capable of owning their own memory, not just containing views into a buffer hosted elsewhere.