fst driver

[wlandau]

An fst driver could be an alternative to #103, and it would be much easier to implement. Benchmarks and setbacks: ropensci/drake#907 (comment).

[wlandau]

And we could still choose the in-memory serialization format based on type/class (re #77 (comment), for Keras models, etc.).

[wlandau]

To be clear, the fst driver could be used for arbitrary data if we save single-column data frames with raw vectors. So its scope, implementation, and usefulness are about the same as the RDS driver.

[wlandau]

Thinking about closing in favor of #110.