Add support for table metadata schema

[molpopgen]

• trying to see if we can make Python-compatible schema. Not looking good so far.
• progress
• rename, b/c we may not stick w/this API
• delete doc test b/c we ain't ready for that yet!
• rename
• rename, part deux
• unbreak the tests
• add a crate, this is probably a dumb idea
• move metadata example program to examples/
• add simplisity python scripts for testing metadata transfer

[molpopgen]

Going to close this one.
We have tests in place for metadata round trips through Python.
The schema do need to be defined in the Python API, as they work through classes defined over there.