Closes#2478
```console
turso>create table t(a,b);
turso>insert into t select 1,2 from generate_series(1,20);
turso>create index idxa on t(a);
turso>create index idxb on t(b);
# segfault
```
The issue was the `turso_ext::Conn` pointer was stored on the
`VirtualTable`, which lives longer than necessary and the underlying
core `Connection` is not guaranteed pinned. Moving the `turso_ext::Conn`
on to the cursor fixes this issue because it's independent of the
Schema.
This also fixes the issue that comes up when that issue is fixed, and
some of the relevant code for this hadn't been updated when other
surrounding changes had been made.
Closes#2479
edd7c45c22