Store Hex username in credentials file and improve key revocation error message

[scristobal]

resolves #4319

Only try to revoke token if new and previous usernames match, otherwise hint the user to delete the token manually.

Summary of changes:

• Store username in credentials file.
• Change credentials file format to TOML.
• Fallback to old format when TOML deserialization fails.

before:

% gleam hex authenticate

You already have a local Hex API token. Would you like to replace it
with a new one? [y/n]: y
https://hex.pm username: B
https://hex.pm password (will not be printed as you type): 

Please enter a new unique password. This will be used to locally
encrypt your Hex API key.

Local password (will not be printed as you type):
Encrypted Hex API key written to .../gleam/hex/hexpm/credentials
Deleting previous key `...` from Hex
error: Hex API failure

There was a problem when using the Hex API.

This was error from the Hex client library:

    an unexpected response was sent by Hex: 404 Not Found: {"message":"Page not found","status":404}

after:

% gleam hex authenticate

You already have a local Hex API key. Would you like to replace it
with a new one? [y/n]: y
https://hex.pm username: B
https://hex.pm password (will not be printed as you type):

Please enter a new unique password. This will be used to locally
encrypt your Hex API key.

Local password (will not be printed as you type):
Encrypted Hex API key written to /Users/samu/Library/Caches/gleam/hex/hexpm/credentials

Your previous Hex API key was created with username `A` which is different from the username
used to create the new Hex API key. You have to delete the key `Mac-1743837627` manually at https://hex.pm

[lpil]

Very nice, thank you. Left some notes inline

[lpil]

We wrap before 80 columns! Could you call the wrap function on this please.

[scristobal]

done!

[lpil]

Same for this also please.

[scristobal]

done!

[lpil]

This silently discards file system errors!

Let's move all of this loading logic into the load function, and have it read once, return an error if the file isn't readable, and also if the file is in neither format.

[scristobal]

Good call, than you for your feedback.

I moved the logic to a separate function as suggested, it does also read better.