Add support for metadata for nodes.

This is useful for retrieving information about some aspect of the
node. A good example is human readable names associated with the node
and it's input and output buses. This is useful for user interfaces
where a brief description of the node such as "Low Pass Filter" can be
drawn on the screen. It's also useful for buses to be named, such as
the source/carrier and excite/modulator on a vocoder effect which would
also need to be visible on a UI.