Fix skips/todos on property tests

[billylanchantin]

We discovered a number of bugs while developing #1012. This issue will keep track of known bugs and our progress on them.

• DataFrame.new!/2 fails often with the :decimal dtype
• Can't print empty DataFrames
  • DF.new([], dtypes: %{a: :integer}) results in 0 columns instead of 1
• Can't print DataFrames in certain scenarios:
  • :millisecond datetimes with high years
  • {:list, {:struct, ...}} columns
  • This panics with assertion failed: i < self.len():

    dtypes = [{"col_k", {:list, {:struct, [{"r", :category}]}}}]
    cols = [{"col_k", [[], [%{"r" => ""}]]}]
    df = DF.new(cols, dtypes: dtypes)

• Can't dump to CSV in certain scenarios
• Can't dump to IPC in certain scenarios
• Can't dump to NDJSON in certain scenarios
• Can't dump to PARQUET in certain scenarios

Feel free to edit this description with updates to the bug-list.

[billylanchantin]

After #1028, we think the only thing left blocking this issue is #1011.

We're currently excluding all dataframes with the :category dtype from the property tests. Once we address #1011, we can remove that exclusion. Unfortunately there may still be additional :category-related issues not related to #1011, but we won't know until it's fixed.