Extend Performance Optimization

Current Extend implementation of the SplitVec is only for convenience rather than for performance. The implementation looks as below

impl<T, G> Extend<T> for SplitVec<T, G>
where
    G: Growth,
{
    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        for x in iter {
            self.push(x);
        }
    }
}

std::vec::Vec implementation, however, uses is optimized with special extend traits. With some book keeping and with the use of size hints, SplitVec can directly call std::vec::Vec::extend method and indirectly utilize these optimizations rather than pushing each element one after the other.

Start by locating the Extend implementation for SplitVec and read how its storage and growth bookkeeping work. Compare it with std::vec::Vec's Extend implementation and size-hint behavior. Done means extending through the underlying Vec operations while preserving SplitVec behavior and improving the current element-by-element approach.