Test API methods (unrelated to memory)

[honno]

Methods such as DataFrame.num_columns() can be easily tested, and so should be. My initial idea is to straight up implement the df protocol partially which uses a dict internally, as really tests are reference implementations anyway. A few days work at most?

There are things like Column.describe_null which can be smoke tested, but testing what they actually return is probably out of scope for this test suite—the roundtrip tests should catch these things, and we can fallback on library authors to debug.