Add a tool for extracting the public key from a signing key

If you have a Ristretto signing key then this gives you a tool for easily finding the public key.

Here's an example:

$ ./result/bin/PaymentServer-get-public-key <ristretto.signing-key 
ogzbCbwIl8qYQkpH1pWoAcJnSpv+UGCHveuEMr471Ew=

Created by: tomprince

Do you need both of these imports?

More or less. getLine to read the private key from stdin as a Data.Text.Text and then putStrLn to write the Data.Text.Text that represents the public key to stdout.

There's probably an entirely different way to manage it but as far as I can tell sticking entirely to Data.Text.IO functions is reasonable.

Created by: tomprince

Do you also need to import the version from Prelude?

import Prelude hiding
  ( putStrLn
  , getLine
  )

has the opposite meaning. It means "add everything from Prelude to this namespace (this is the default if you don't do anything) except putStrLn and getLine". So this is (more or less) necessary to avoid having the next import make these two names ambiguous.

The Prelude import could be dropped but then these two names have to be referred to qualified (eg Data.Text.IO.getLine) to avoid the ambiguity.

Created by: tomprince

Review: Approved

The code looks reasonable, though I've not tested it.