Interpolated strings and SQL

[Nickolay Bukreyev]

I’ve just realized DIP1036 has an excellent feature that is not 
evident right away. Look at the signature of `execi`:

```d
auto execi(Args...)(Sqlite db, InterpolationHeader header, Args 
args, InterpolationFooter footer) { ... }
```

`InterpolationHeader`/`InterpolationFooter` _require_ you to pass 
an istring. Consider this example:

```d
db.execi(i"INSERT INTO items VALUES ($(x))".text);
```

Here, we accidentally added `.text`. It would be an SQL 
injection… but the compiler rejects it! `typeof(i"...".text)` is 
`string`, and `execi` cannot be called with `(Sqlite, string)`.