Can't pass `SqlExpr (Entity e)` to functions

Consider this query:

```sql
SELECT JSONB_AGG(foo)
FROM foo
```

This gets all the `foo`s, converts them to JSON, and stuffs them in a list. We might try to write this as:

```haskell
jsonbAgg
:: SqlExpr (Entity a)
-> SqlExpr (Value [Entity a])
jsonbAgg = unsafeSqlFunction "jsonb_agg"
```

But this fails, because `unsafeSqlFunction` is written only for `Value`s. There's even a catch-all instance that forces a Value:

```haskell
instance (a ~ Value b) => UnsafeSqlFunctionArgument (SqlExpr a) where
toArgList = (:[]) . veryUnsafeCoerceSqlExprValue
```