base_convert: Find an allocation free method. Maybe extract from the top?

[github-actions]

On 2023-08-13 @prestwich wrote in 55c5411 “Merge pull request #293 from recmo/remco/base-to”:

Find an allocation free method. Maybe extract from the top?

    ///
    /// # Panics
    ///
    /// Panics if the base is less than 2.
    #[inline]
    #[cfg(feature = "alloc")] // OPT: Find an allocation free method. Maybe extract from the top?
    pub fn to_base_be(&self, base: u64) -> impl Iterator<Item = u64> {
        struct OwnedVecIterator {
            vec: alloc::vec::Vec<u64>,
        }

From src/base_convert.rs:70