docs: def hash_from_func(cls, func):
          """Return a hashlib-compatible object for the multihash `func`.
  
          If the `func` is registered but no hashlib-compatible constructor is
          available for it, `None` is returned.  If the `func` is not
          registered, a `KeyError` is raised.
  
          >>> h = FuncReg.hash_from_func(Func.sha2_256)
          >>> h.name
          'sha256'
          """
          new = cls._func_hash[func].new
          return new() if new else None