Support columns with non-nullable data types

[caseycrogers]

Currently all columns have nullable data types. If a user has data that explicitly should not be null, this makes their code more verbose (they have to add ! everywhere) and doesn't enforce non-null input.

To support non-nullable data types, we should add a non-nullable column and/or non-nullable DataFrame.

[synw]

Good idea. Should this be done before the next big release on pub with null safety? Should we publish or wait?

[caseycrogers]

I haven't thought too hard about how to implement this yet, but I think it's going to be a bit messy and painful. Currently, we're blocking anyone who wants to update their entire app to null safety-they can't update until df supports null safety. IMO we should publish ASAP to unblock people and then add a non-nullable DataFrame later.

[synw]

0.4.0-nullsafety.0 is published on pub