Weaken `AlgebraicValue`'s `Hash` function to ignore enum discriminants

[Centril]

The current implementation of AlgebraicValue is #[derive(Hash)].
This causes the hash function to hash like so for e.g., AlgebraicValue::U16(x) to be:

hasher.write_u8(8); // The discriminant of `AlgebraicValue::U16` is 8.
hasher.write_u16(x); // The actual value `x: u16` in `::U16`.

Instead, based on my discussion with @gefjon, we agreed that, we should only hash each AV's payload and not the tag/discriminant of the AV variant itself. The benefits include:

1. This weakening should be more performant because its less work.
2. This is what the hash function for RowRef already does.
3. It's easier to define a hash function for BSATN-encoded rows this way.

The weakening should not introduce more collisions because AVs in e.g., subscriptions follow a schema, so when you have 100 ::U8s, it does not reduce the rate of collisions when you hash the tag of U8.