Add TableCollection functions to consume "row" objects

[molpopgen]

Named arguments are handy. Rust only really allows them for struct construction. So, we can "fake it" with an API like:

tables.add_node_table_row(NodeTableRow{ flags: 0, time: 0.1243, population: 2, individual: TSK_NULL, metadata: None }).unwrap()

Just have to check what the visibility of the row objects is. They are public, but maybe not currently from the crate root?

This is certainly undesirable to do "everywhere", but it is handy for the row adding fxns that are a bit long. The other functions that are long on the Python side are often shorter on the C side to to bit flags.