merkle_tree: let bulk_build/3 hash the tree
merkle_tree: let bulk_build/3 hash the tree

this saves an extra traversal compared to running gen_hash/1 afterwards
tectronics/scalaris,seyyed/scalaris,digshock/scalaris,tectronics/scalaris,seyyed/scalaris,tectronics/scalaris,digshock/scalaris,tectronics/scalaris,seyyed/scalaris,tectronics/scalaris,seyyed/scalaris,digshock/scalaris,tectronics/scalaris,seyyed/scalaris,seyyed/scalaris,digshock/scalaris,tectronics/scalaris,digshock/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris