Phpstan level 8

[AJenbo]

Analyze the code with PHPStan to find potential issues.

I have split this up in to several PRs to hopefully make it easier to evaluate, debate and adjust each needed change. Alternatively you can review and merge this PR the others can be closed.

Level 8 depends on:

• Correct signature of Trustly_Data::json() #51
• Narrow validation of curl_exec() #50

Level 6 depends on:

• Add return type to Trustly_Api::call() #49
• Correct return type of Trustly_Data::set() #48
• Add missing types #47

Level 5 depends on:

• Remove redundant cast #45
• Correct signature of Trustly_Data::useMerchantPrivateKey() #44
• Fix error message for unknown cURL errors #43
• Use defined() to check if const is defined #42

Level 3 depends on:

• Make Trustly_Api_Unsigned::call() compatible with Trustly_Api::call() #41
• Correct signature of signMerchantRequest #46
• Correct decleration of properties #39

Level 2 depends on:

• Corrrect signature of Trustly_Api::verifyTrustlySignedResponse() #38
• Correct signature of Trustly_Api::serializeData() #37
• Correct missing return types #36
• Correct args where the name was not specificed correctly #35
• Correct signature of nullable arguments #34

Level 1 depends on:

• Fix error message on openssl_error_string() failure #33
• Correct number of given argument #32
• Correct return type for various methodes with null return #31

Level 0 depends on:

• Add missing properties #28
• Correct signature for setSignature() #27
• Check that files are valid for all supported versions #26