Release v0.5.6

rework behavior of empty_let_in, thanks to a report from @SuperSandro2000:

• if the let-in expression consists of only comments, statix check raises a warning, but statix fix leaves it as it is, because the comments may be commented out code
• if the let-in expression is completely empty (no comments, no bindings, no inherits), statix check will raise a warning, statix fix will apply the fix