Extract module SafeAssignment.

With functionality for safe assignment checks.
Include the module CheckMethods in more places.
Move handling of :match_current_line nodes to module IfNode.
[Fix #709] Handling of class vars and multi-assign in EndAlignment.
Fix missing handling of class vars in SpaceAroundOperators.
Include module CheckAssignment from BlockAlignment.
Merge pull request #710 from jonas054/refactor_duplication

Refactor duplication