Add line_after_use fixer

Implements #153
Replaces #800 
How is it the same? 3rd case is a test for multiple use statements, which is important since there must be blank line afer use block, hence i was testing that is correctly done when having one and two use statements. I could perhaps drop this one and only keep the 3rd, but i dont think the extra testcase hurts?
Especially is someone changes the code in the future, things might go wrong, by just adding blankline after any use statement, which is not correct if there is another use.