Fill is a static factory function it should be moved to Chain

[nreynis]

Fill is actually a Trait.
https://github.com/cocur/chain/blob/master/src/Link/Fill.php

This is the only Trait that don't operate on an instance and also the only one that is static.
Other Traits are all instance methods, this one is a factory.

This code should be moved to the Chain class which host other factory methods create and createFormString. This would be an entirely transparent change, it would make the code more consistent and would prevent statistical analysis failing:

https://scrutinizer-ci.com/g/cocur/chain/inspections/fba87ae0-dc03-4190-966a-64fd5f908b9b/issues/files/src/Link/Fill.php?status=all&orderField=path&order=asc&honorSelectedPaths=0

[florianeckerstorfer]

Fixed in #49