[nexus] Experiment with using closer-to-raw-SQL pagination

[smklein]

The pagination functions - in particular, the paginated_multicolumn function - is notorious
for having incredibly complex generic parameters.

For context, see:

omicron/nexus/db-queries/src/db/pagination.rs

Lines 82 to 177 in feae60c

	pub fn paginated_multicolumn<T, C1, C2, M1, M2>(
	query: T,
	(c1, c2): (C1, C2),
	pagparams: &DataPageParams<'_, (M1, M2)>,
	) -> <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output
	where
	// T is a table^H^H^H^H^Hquery source which can create a BoxedQuery.
	T: QuerySource,
	T: AsQuery,
	<T as QuerySource>::DefaultSelection:
	Expression<SqlType = <T as AsQuery>::SqlType>,
	T::Query: query_methods::BoxedDsl<'static, Pg>,
	// Required for...everything.
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output: QueryDsl,
	// C1 & C2 are columns which appear in T.
	C1: 'static + Column + Copy + ExpressionMethods,
	C2: 'static + Column + Copy + ExpressionMethods,
	// Required to compare the columns with the marker types.
	C1::SqlType: SqlType,
	C2::SqlType: SqlType,
	M1: Clone + AsExpression<C1::SqlType>,
	M2: Clone + AsExpression<C2::SqlType>,
	// Necessary for `query.limit(...)`
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::LimitDsl<
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	// Necessary for "query.order(c1.desc())"
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::OrderDsl<
	Desc<C1>,
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	// Necessary for "query.order(...).then_order_by(c2.desc())"
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::ThenOrderDsl<
	Desc<C2>,
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	// Necessary for "query.order(c1.asc())"
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::OrderDsl<
	Asc<C1>,
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	// Necessary for "query.order(...).then_order_by(c2.asc())"
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::ThenOrderDsl<
	Asc<C2>,
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	// We'd like to be able to call:
	//
	// c1.eq(v1).and(c2.gt(v2))
	//
	// This means "c1.eq(v1)" must implement BoolExpressionMethods, and
	// satisfy the requirements of the ".and" method.
	//
	// The LHS (c1.eq(v1)) must be a boolean expression:
	Eq<C1, M1>: Expression<SqlType = Bool>,
	// The RHS (c2.gt(v2)) must be a boolean expression:
	Gt<C2, M2>: Expression<SqlType = Bool>,
	// Putting it together, we should be able to filter by LHS.and(RHS):
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::FilterDsl<
	And<Eq<C1, M1>, Gt<C2, M2>>,
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	// We'd also like to be able to call:
	//
	// c1.eq(v1).and(c2.lt(v2))
	//
	// We've already defined the bound on the LHS, so we add the equivalent
	// bounds on the RHS for the "Less than" variant.
	Lt<C2, M2>: Expression<SqlType = Bool>,
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::FilterDsl<
	And<Eq<C1, M1>, Lt<C2, M2>>,
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	// Necessary for "query.or_filter(c1.gt(v1))"
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::OrFilterDsl<
	Gt<C1, M1>,
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	// Necessary for "query.or_filter(c1.lt(v1))"
	<T::Query as query_methods::BoxedDsl<'static, Pg>>::Output:
	query_methods::OrFilterDsl<
	Lt<C1, M1>,
	Output = <T::Query as query_methods::BoxedDsl<'static, Pg>>::Output,
	>,
	{

To reduce this pain, this PR provides pagination helpers which side-step Diesel's type-checking,
aiming to provide a more flexible option for pagination which can work across arbitrary JOINs,
UNIONs, etc, without relying on compile-time type-checking.

[smklein]

To make the RawPaginator API with the interface that exists, these functions needed to act on &mut self instead of self.

This was a breaking change, and it requires using an Option<BoxQuery> internally, but that inner type detail should not be visible to callers.

[smklein]

It seems way for feasible to make this generic across a tuple of an arbitrary length -- the generic parameters are much simpler.

[smklein]

I picked the affinity "list" APIs because those couldn't use the old pagination functions anyway - they had to be manual, to paginate over a UNION subquery.

I've converted everything in this file to use the RawPaginator.

[smklein]

This file also uses a combination of paginated and paginated_multicolumn.

I picked it as a case where "there are a lot of joins + filters", to see if the new helpers can work.