add return types

[tacman]

This seems to pass some static tests, but I'm not sure how to set the API key to do all the unit tests.

It just gets the return type from the annotations and uses that.

[tacman]

@samwilson can you take a look at these issues? I can probably fix them, but perhaps they can be slightly refactored to return null instead of false.

Also, is oAuth1 still supported? How should it be handled?

src/PhpFlickr.php:338 PhanTypeMismatchReturnSuperType Returning $accessToken of type \OAuth\Common\Token\TokenInterface but retrieveAccessToken() is declared to return \OAuth\OAuth1\Token\TokenInterface|\OAuth\OAuth2\Token\TokenInterface|string (saw a supertype instead of a subtype)
src/TestApi.php:16 PhanTypeMismatchDeclaredReturnNullable Doc-block of testEcho has declared return type bool which is not a permitted replacement of the nullable return type array|bool|null declared in the signature ('?T' should be documented as 'T|null' or '?T')
src/TestApi.php:16 PhanTypeMismatchDeclaredReturnNullable Doc-block of testEcho has declared return type string[] which is not a permitted replacement of the nullable return type array|bool|null declared in the signature ('?T' should be documented as 'T|null' or '?T')
[debug] Restarted process exited 1
Script phan --allow-polyfill-parser --load-baseline .phan/baseline.php handling the phan event returned with error code 1
Script @phan was called via test

[samwilson]

OAuth 1 is the only version that's supported, I think?

This PR doesn't have any changes, by the way.

I'll start working through the Phan issues to improve things. e.g. #86.

[samwilson]

But yes, I think in general we can switch returns from string[]|bool to ?string[]. Will that be another major version bump? :-) (I think it will but that's fine.)