Add postfix increment/decrement operator

[LefterisJP]

At the moment we only have a syntactic prefix increment and decrement operator ++ and -- which acts like a postfix operator.
Think on whether we also want the postfix one and if yes then properly implement the different behaviour in the backend too.

Remember that the rule is:

• Postfix operator evaluates the value of the expression prior to incrementing or decrementing
• Prefix operator evaluates the value of the expression after incrementing or decrementing.

At the moment in the backend we simply increment or decrement one to the expression's value after it has been evaluated.