[nexus] Make 'update_and_check' CTE explicitly request columns

[smklein]

Related to #4570 , but not a direct fix for it

This PR removes a usage of ".*" from a SQL query. Using ".*" in sql queries is somewhat risky -- it makes an implicit dependency on order, and can make backwards compatibility difficult in certain circumstances.

Instead, this PR provides a ColumnWalker, for converting a tuple of columns to an iterator, and requests the expected columns explicitly.

[smklein]

I would like to turn this into the following

Suggested change

	// TODO: don't convert to vec? You'll need to figure
	// out how to state the type of IntoIter.
	[$($column::NAME,)+].to_vec().into_iter()
	[$($column::NAME,)+].into_iter()

... but this would require changing:

type IntoIter = std::vec::IntoIter<Self::Item>;

Into:

type IntoIter = std::array::IntoIter<Self::Item, N>;

I don't know how to specify the type of "N" here, even though it should be knowable -- it's the length of $column.

[sunshowers]

I think either of the first two options in https://danielkeep.github.io/tlborm/book/blk-counting.html would be fine. A better approach would be rust-lang/rust#83527 which isn't stable yet.

[sunshowers]

Actually in this case you can just pass in the length as an argument to impl_column_walker.

[smklein]

Ah, good call, I'll pass the length explicitly.

[sunshowers]

Looks great!