Controlled casting

[mathemancer]

Fixes #126

This adds a number of DB functions, and replaces the column alteration logic to use them. These functions give us more precise control over what is done when casting one type to another (mostly to make things safer).

Technical details

This requires knocking out your Mathesar DB and reinstalling, since we still don't have upgrade functionality working.

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• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[kgodey]

@mathemancer I added some comments but I'm approving to unblock merge. If I'm around when you're done making changes, feel free to ask me to rereview before merge.

[kgodey]

Datetimes should be fairly simple to add too, right?

[mathemancer]

I'd rather leave those for a separate issue / PR, since there'll be some complexity around dates vs times vs datetimes, and TZs (how to decide whether a column of times should be imported with the local TZ, assuming UTC, etc.)

[mathemancer]

I know we've discussed the plan w.r.t. TZs, but I think having dates/times/datetimes separated out in a well-documented PR might benefit future code archaeologists.

[kgodey]

Makes sense.

[kgodey]

Could you document what this function (and other similar functions) are doing in comments?

[mathemancer]

Good idea.

[mathemancer]

I documented them with docstrings to make it visible using the help function. I also considered documenting them in comments in the PL/pgSQL functions themselves instead, but I think folks are more likely to be able to (or want to) read the python docstrings than comments in the SQL. Do you agree?

[kgodey]

Yes, agreed.

[kgodey]

Looks great, @mathemancer

[kgodey]

Makes sense.