Function grin_chain::txhashset::rewindable_kernel_view

pub fn rewindable_kernel_view<F, T>(
    trees: &TxHashSet, 
    inner: F
) -> Result<T, Error> where
    F: FnOnce(&mut RewindableKernelView) -> Result<T, Error>, 

Rewindable (but still readonly) view on the kernel MMR. The underlying backend is readonly. But we permit the PMMR to be "rewound" via last_pos. We create a new db batch for this view and discard it (rollback) when we are done with the view.