TableBuilder: Make a new type for columns

Instead of making the columns a hash where the types get all confused
and mussed and we have to type check wherever we use the column value,
make the columns into their own real type.

Adds a new `Column` class to the helper module which lets us interact
with table column data in a much more natural way. Here, when mapping
over the column list, instead of getting a key and value which could be
whatever, we get a real `Column` object that we can call methods on to
get the appropriate data.

It's still not perfect, as evidenced by the `#tbody` method, which has
remained largely the same, but at least we're not checking
`attribute.is_a?(Hash)` like I had tried before.

Update the tests to use the new `TableBuilderHelper::Column` syntax for
defining columns.

Refs #3479

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