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.

sqlite3/src/lib.rs

@@ -572,7 +572,6 @@ pub unsafe extern "C" fn sqlite3_value_type(value: *mut ffi::c_void) -> ffi::c_i
         limbo_core::OwnedValue::Float(_) => 2,
         limbo_core::OwnedValue::Text(_) => 3,
         limbo_core::OwnedValue::Blob(_) => 4,
-        _ => unreachable!(),
     }
 }