Trait for auto-inc sentinel values

[kazimuth]

Currently we use 0 all over the place in the codebase, implicitly because it is the sentinel value that means "replace me when I am inserted in to a table with a sequence defined on my column". We should make a trait for this. I would propose something like:

trait AutoIncrementable {
   /// A constant that will be replaced with a unique value upon insertion into a column with the `auto_inc` attribute.
   const SENTINEL: Self;
}

or something like that in spacetimedb_lib. This can be used in the client SDK, module SDK, and in the core database for e.g. SystemIDs.

Note that "auto-inc" and "sequence" are used as synonyms throughout the codebase. We should probably also settle on one of these for consistency of documentation...

[Centril]

There is a trait in crates/bindings/src/table.rs:

/// A trait for types that know if their value will trigger a sequence.
/// This is used for auto-inc columns to determine if an insertion of a row
/// will require the column to be updated in the row.
///
/// For now, this is equivalent to a "is zero" test.
pub trait IsSequenceTrigger {
    /// Is this value one that will trigger a sequence, if any,
    /// when used as a column value.
    fn is_sequence_trigger(&self) -> bool;
}

[gefjon]

What implementations of is_sequence_trigger or values of SENTINEL other than the integer 0 would we want? I have a hard time imagining any case where we'd want to allow #[auto_inc] on non-integral types.

[Centril]

My primary motivation for the trait was to have something reliable to use for type checking purposes, as opposed to the heuristic check.