Support struct access in slices in where clauses

Consider:

```go
type machine struct {
UUID string `db:"uuid"`
}
query, err := sqlair.Prepare(`
SELECT uuid AS &machine.uuid FROM machine;
`)
...
var machines []machine
db.Query(ctx, query).Get(&machines)
```

Now later on, we want to use the same information to update a set of columns. We now need to transform the slice of `[]machine` to `[]string` to be useful.

```go
s := make(sqlair.S, len(machines))
for i, v := range machines {
s[i] = v.uuid
}
update, err := query, err := sqlair.Prepare(`
UPDATE machine WHERE uuid IN ($S[:])
`, sqlair.S{})
...
db.Query(ctx, update, s)
```

Instead, it would be preferable to do the following:

```go
update, err := query, err := sqlair.Prepare(`
UPDATE machine WHERE uuid IN ($machines[:].uuid)
`, machines)
...
db.Query(ctx, update, machines)
```

Start by tracing the existing handling of `$S[:]` and struct field mapping in the SQLair implementation. Verify how a `[]machine` is bound for `WHERE uuid IN (...)`, then add coverage showing that `$machines[:].uuid` accepts the example slice and produces the intended update query.

Written by the indexing model from the issue text.