result streaming

[jraymakers]

Implement several "deprecated" C API functions related to result streaming. While these are deprecated, the functionality they expose is likely to continue to exist in some form in the future.

New functions:

• row_count
• result_get_chunk
• result_is_streaming
• result_chunk_count
• execute_prepared_streaming
• pending_prepared_streaming

Also add null as a possible type to some existing C API type definitions (because the underlying function can return null):

• The validity argument to validity_row_is_valid.
• The result of fetch_chunk.