* 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
* Add mnenomic keypair generation and recovery to cli
* Use password input to retrieve mnemonic phrase
* Direct users without keypair file to use solana-keygen