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/go/rs_src/types.rs

@@ -160,7 +160,6 @@ impl LimboValue {
             limbo_core::OwnedValue::Null => {
                 LimboValue::new(ValueType::Null, ValueUnion::from_null())
             }
-            _ => unreachable!(),
         }
     }