Export schema from data objects

[kylebarron]

Closes #53.

In apache/arrow#39689 the consensus seems to be that objects exporting __arrow_c_stream__ should also export __arrow_c_schema__ so that data consumers can

1. see the Arrow schema of the producer's data
2. decide whether to ask for data with a different Arrow schema
3. call __arrow_c_stream__ with that other Arrow schema if desired