Feature request: `insert_or_update` ("upsert")

Something like this:
```
// Pseudocode
func upsert(ctx, primary_key_value, value):
  if ctx.primary_key_column.try_find(primary_key_value):
    // Row with a matching primary key column exists
    ctx.primary_key_column.delete(primary_key_value)
  ctx.primary_key_column.insert(primary_key_value, value)
  
```

In order to `upsert` on a column it needs to have a unique constraint on it.