Update Hook::safe_offset() to handle closures

[unfulvio-godaddy]

Summary

Reimplements #174 after implementing PhpStan and code styles in WP_Mock. Tests have been reworked in PhpUnit.

Quote from OP:

WP_Mock\Functions::type('callable') was causing a mismatch between the safe_offset values returned for the Mockery matcher and the actual closure being passed to add_action. The matcher was getting spl_object_hash'd, while the Closure was getting "__CLOSURE__". Now, safe_offset() treats Mockery matchers with type closure as "__CLOSURE__" too.

Closes: #89

Test

• Unit tests pass

Open questions

I think we could rename safe_offset() into something more meaningful to make it easier to understand what it does. Seems this is used solely within the Hook abstract and Filter/Action implementations to get a string representations of the hook arguments. We could rename it using camel case and soft deprecate the former method.

As predicted, PhpStan will start spitting errors as tests, return types and such start to change. I covered some of the issues in this PR, but had to update the baseline to fix several unrelated positives which we can address in subsequent PRs.

[coveralls]

Coverage: 18.331% (+3.6%) from 14.743% when pulling 341b2a6 on feature/support-closures-anonymous-functions into 9d16132 on trunk.