feat(smt/bpt): Adds SaveSnapshot and LoadSnapshot features

Closes AC-983 "Add functions to smt for duplicating a bpt"

Adds two functions to the BPT SaveSnapshot LoadSnapshot

These functions take a file name and, naturally, save or load a snapshot. The value part is up in the air... The assumption is that the hash in the BPT is a key to something in the database that can be loaded to represent the state of the behind the bpt entry's account.

More work may be needed to tune that part of the snapshot.

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