Function grin_chain::txhashset::header_extending

pub fn header_extending<'a, F, T>(
    trees: &'a mut TxHashSet, 
    batch: &'a mut Batch, 
    inner: F
) -> Result<T, Error> where
    F: FnOnce(&mut HeaderExtension) -> Result<T, Error>, 

Start a new header MMR unit of work. This MMR tracks the header_head. This MMR can be extended individually beyond the other (output, rangeproof and kernel) MMRs to allow headers to be validated before we receive the full block data.