* SDK: Refactor (read|write)_keypair
Split file opening and data writing operations
Drop filename == "-" stdio signal. It is an app-level feature
* keygen: Move all non-key printing to stderr
* keygen: Adapt to SDK refactor
* keygen: Factor keypair output out to a helper function
* Migrate from ring to ed25519-dalek
* Move gen_keypair_file test to a more appropriate location
* Fixup bench-exchange and add helper fn for single deterministic keypair
* Update golden