Consider making SmallInteger a wrapper over SmallBigInt, not vice versa

[andreubotella]

Currently it is possible to do something like Value::Integer(SmallInteger::try_from(SmallInteger::MAX_BIGINT).unwrap()), which does not panic and would result in an invalid value (because SmallInteger::MAX_BIGINT is outside the range of a safe integer).

To avoid this, we should probably make the base type SmallBigInt, since its valid range is larger, and make SmallInteger a wrapper over it that checks the range at construction.