Generalize `elif-else` to all compound statements

Ideas
28

To put it together:

elif-else clause combination is entered when a certain condition in the initial statement-clause construct is NOT satisfied.

These conditions are different for different cases: if, while & for and try-except.

  1. if: elif-else is entered if condition in if condition is NOT satisfied
  2. while & for: elif-else is entered if NO break was executed in the loop body
  3. try-except: elif-else is entered if NO exceptions were raised in the try body