Rename refactoring should check whether the target variable exists

[lieryan]

The following series of tickets all have the same root cause, rope currently doesn't validate whether the target variable name is currently occupied by another existing variable:

• Rename Method refactoring allows you to rename a nested method with a parameter name #776
• Rename Method refactoring allows you to rename a method to a name with an 'internal use' indicator #769
• Rename Method refactoring allows the use of previously declared field name #768
• Rename Field refactoring allows you to rename a local field with the method name #766
• Rename Field refactoring allows you to rename a class field with class method names #765
• Rename Field refactoring allows the use of a name that can not be iterable #764
• Rename Field refactoring allows the use of declared method names as new field names #763
• Rename Field refactoring allows you to rename a field with the same name used in a global method #761

Collecting here into one big ticket since most of them will likely be resolved by the same solution.

Once this ticket is resolved, we'll go through each ticket to verify that those cases are addressed by the solution.