Sparse_ledger.of_ledger_subset_exn shouldn't ever compute hashes

[emberian]

Every hash it needs is already right there! here is the call to add_path that forms the core of of_ledger_subset_exn, and here is the definition of add_path. Note the calls to Hash.merge - these are expensive and wasteful. Instead, we could augment the notion of path to include the merge hashes.

I've started some work on the fix/no-sparse-ledger-hashing branch that does this. merkle_path_full needs to be implemented for all the relevant ledgers.

[emberian]

Done by #14594