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/javascript/src/lib.rs

@@ -101,7 +101,6 @@ fn to_js_value(env: &napi::Env, value: &limbo_core::OwnedValue) -> JsUnknown {
         limbo_core::OwnedValue::Blob(b) => {
             env.create_buffer_copy(b.as_ref()).unwrap().into_unknown()
         }
-        _ => env.get_null().unwrap().into_unknown(),
     }
 }