Support for multi-index uniqueness constraints

[joamaki]

Consider the following:

type Object struct {
  A string
  B string
}

var (
  AIndex = index[...]{
    FromObject: ... return index.NewKeySet(index.String(obj.A)) ...
    Unique: true,
  }
  BIndex = index[...]{
    FromObject: ... return index.NewKeySet(index.String(obj.B)) ...
    Unique: true,
  }
}

func NewObjectTable(db *DB) (RWTable[Object], error) {
  return NewTable(db, "objects", AIndex, BIndex)
}

var tbl RWTable[Object]
wtxn := db.WriteTxn(tbl)
tbl.Insert(wtxn, Object{"A", "B"})
tbl.Insert(wtxn, Object{"B", "B"})
wtxn.Commit()

Following the inserts the indexes now contain:

AIndex = {"A": Object{"A", "B"}, "B": Object{"B", "B"}}
BIndex = {"B": Object{"B", "B"}}

We've now silently lost the association from "B" to Object{"A", "B"}. For most use-cases this is a bug and the fact that this overriding happens silently could be a foot-gun. The one use-case where I could see this behavior being useful is to have a "latest" index.

Consider adding support for validating that an entry in a secondary unique index can only be replaced if the primary keys match.
Since Insert errors are not often checked currently (they're currently exceptional errors such as trying to insert with a WriteTxn that doesn't lock the table), also consider adding support for configuring the handling for errors to allow panicing in CI tests and logging errors in production.