Introduce Register struct

OwnedValue has become a powerhouse of madness, mainly because I decided
to do it like that when I first introduced AggContext. I decided it was
enough and I introduced a `Register` struct that contains `OwnedValue`,
`Record` and `Aggregation`, this way we don't use `OwnedValue` for
everything make everyone's life harder.

This is the next step towards making ImmutableRecords the default
because I want to remove unnecessary allocations. Right now we clone
OwnedValues when we generate a record more than needed.

bindings/python/src/lib.rs

@@ -333,7 +333,6 @@ fn row_to_py(py: Python, row: &limbo_core::Row) -> Result<PyObject> {
             limbo_core::OwnedValue::Blob(b) => {
                 py_values.push(PyBytes::new(py, b.as_slice()).into())
             }
-            _ => unreachable!(),
         }
     }
     Ok(PyTuple::new(py, &py_values)