This commit moves the schemas used for slow-query benchmarks into a
separate SQL file so that the schemas can continue to grow without
making a mess of `bench_test.go`.

Release note: None

Release note: None

`BenchmarkSlowQueries` now runs each slow query benchmark with
`reorder_join_limit` set to `0` and `8`. This will allow us to analyze
the range of query optimization performance with respect to this setting
which is commonly adjusted to reduce planning latency.

Release note: None

Prior to this commit, `FuncDepSet.tryToReduceKey` was called at the end
of `FuncDepSet.addEquivalency`. This was inefficient because
`FuncDepSet.AddFrom` and `FuncDepSet.AddEquivFrom` can call
`addEquivalency` multiple times, and the FD's key only needs to be
reduced after all equivalencies and dependencies have been added. Now,
`tryToReduceKey` is no longer called in `addEquivalency` and callers are
responsible for ensuring that it is called. This mimics the behavior of
the similar function `addDependency`.

Release note: None

This commit adds a new method `FuncDepSet.addConstantsNoKeyReduction`
which is the same as `FuncDepSet.AddConstants`, only differing because
it not attempt to reduce the key like `AddConstants does. This allows us
to avoid key-reducing computation in a few places.

Release note: None

This commit changes the invocation of `ComputeEquivClosure` within
`FuncDepSet.addEquivalency` to `ComputeEquivClosureNoCopy`. This is
valid because `addEquivalency` is only called with sets that have
already been copied, so mutating the input set is safe.

Release note: None

This commit reduces allocation in `statisticsBuilder.colStatJoin`.
Previously, it was creating intersections of two sets, which, in some
cases, were only useful for checking their emptiness. Now we use the
`ColSet.Intersects` method which returns a boolean and does not
build a new set.

Release note: None

This commit reduces allocations in the statistics builder by avoiding
creating singleton column sets. A new `ColStatsMap.LookupSingleton`
method allows columns stats for a single column to be fetched without
building a column set and potentially allocating memory.

Release note: None