Static closures should be employed whenever `$this` is not used

[westonruter]

Bug Description

There closures being used in the codebase which do not use $this. Granted, in most cases clases aren't even involved, but in those that are, using static is better for memory usage since the $this reference can be garbage collected. See SO answer.

I suggest that we do the following:

1. Add static to all closures that don't use $this.
2. Incorporate the SlevomatCodingStandard.Functions.StaticClosure sniff to catch this automatically.