static BiFunction<Type, ConfigRegistry, ColumnMapper<?>> singleton(ColumnMapper<?> instance) {
        return (t, c) -> instance;
    }
Column mapper factory which knows how to map Optionals:
<ul>
    <li>{@link Optional}</li>
    <li>{@link OptionalInt}</li>
    <li>{@link OptionalLong}</li>
    <li>{@link OptionalDouble}</li>
</ul>
core/src/main/java/org/jdbi/v3/core/mapper/OptionalColumnMapperFactory.java