Add BSER load/dump support for variants #1

BSER is a local-IPC only serialization format for dynamic data:
https://github.com/facebook/watchman/blob/master/BSER.markdown

Adding support for this to Phenom requires adding load/dump APIs similar to the JSON load/dump API.

It also means adding API support for "Array of Templated Objects". In Watchman, this is achieved simply by storing the template definition as a special property of the object type.