Rename and possibly refactor the inner types of IterByColRange

[bfops]

pub enum IterByColRange<'a, R: RangeBounds<AlgebraicValue>> {
    /// When the column in question does not have an index.
    Scan(ScanIterByColRange<'a, R>),

    /// When the column has an index, and the table
    /// has been modified this transaction.
    Index(IndexSeekIterInner<'a>),

    /// When the column has an index, and the table
    /// has not been modified in this transaction.
    CommittedIndex(CommittedIndexIter<'a>),
}

The types here could have better names. Also does it need to be explicitly parameterized with a range or should it agnostic to the type of index?

[bfops]

Would this affect the API at all?