This allows us to search for sets of items by multiple values in a
single query, e.g. a query semantically equivalent to:

```sql
select from things
  where (a,b,c) in ((?,?,?), ...)
```

or the expanded form:

```
select from things
  where ((a = ? and b = ? and c = ?)
         or
         (a = ? and b = ? and c = ?)
         ...
         )
```

This is provided by a concern `WhereComposite` which allows us to search
in this way.

We support structs as well as hashes as input.