Code improvements (#1649)

* use global namespace for faster lookups
* phpstan level 5

tests/Invoice/Calculator/ActivityInvoiceCalculatorTest.php

@@ -149,7 +149,7 @@ class ActivityInvoiceCalculatorTest extends AbstractCalculatorTest
         $this->assertEquals('EUR', $model->getCurrency());
         $this->assertEquals(2521.12, $sut->getSubtotal());
         $this->assertEquals(6600, $sut->getTimeWorked());
-        $this->assertEquals(5, count($sut->getEntries()));
+        $this->assertEquals(5, \count($sut->getEntries()));
 
         $entries = $sut->getEntries();
         $this->assertEquals(404.38, $entries[0]->getRate());