Provide a way to (de)serialize metas

[dlurton]

Currently, metas associated with an IonElement are lost when the element is round-tripped through Ion binary or text formats and back again. Thus, it is up to the application to provide a way to persist metas, if the application cares about them, but this library probably should provide some assistance here--such as APIs to specify functions that (de)serialize metas.

element_with_metas::(
     (lit 42) --the original node
     { '$source_location': (1, 1) } -- the meta information associated with the original node.
)

[dlurton]

Oops, duplicates #20