Allow selecting just one column

```rust
if let Some(rowid) = select!(Option<_turbonet_peers.rowid> "WHERE ip = " ip)? {
```
vs
``` rust
if let Some(_turbonet_peers { rowid, .. }) = select!(Option<_turbonet_peers> "WHERE ip = " ip)? {
```

I think this is also why we have that detailed columntype info in the migrations.toml