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/java/rs_src/limbo_statement.rs

@@ -116,7 +116,6 @@ fn row_to_obj_array<'local>(
             }
             limbo_core::OwnedValue::Text(s) => env.new_string(s.as_str())?.into(),
             limbo_core::OwnedValue::Blob(b) => env.byte_array_from_slice(&b)?.into(),
-            _ => unreachable!(),
         };
         if let Err(e) = env.set_object_array_element(&obj_array, i as i32, obj) {
             eprintln!("Error on parsing row: {:?}", e);