load/save hashes of metadata

load/save hashes of metadata

This commit reorganizes how the persist code treats hashing. The idea is
that each crate saves a file containing hashes representing the metadata
for each item X. When we see a read from `MetaData(X)`, we can load this
hash up (if we don't find a file for that crate, we just use the SVH for
the entire crate).

To compute the hash for `MetaData(Y)`, where Y is some local item, we
examine all the predecessors of the `MetaData(Y)` node and hash their
hashes together.