Remove the `having` function, incorporate into `select`

The `having` function duplicated the intended functionality of `select`, but in
a bad way. It only applied to a restricted case, and its existence meant that
`select` provided an escape-hatch with which we could cause some unexpected
behaviour. By consolidating the two functions into `select` we remove the
escape hatch as well as simplify the model.

Selection on grouped queries may now introduce a new subquery, but only in
situations where the behaviour is unpredictable (ie. selecting on non-grouped
attributes).

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