feature: add support for signaling that an expression can be wrapped into a method call

(from @oscarlvp)

`x = i` can be transformed into `x=f(i)`.
`i++` cannot be transformed into `f(i)++`.

In CtUnaryOperator, `getOperand()` has a generic role `EXPRESSION` and getRole() cannot be used for checking that an expression can be wrapped into a method call

The solution is not obvious.

* changing the role of `getOperand()` from EXPRESSION to ASSIGNED. But this does not work for all CtUnaryOperator
* adding a method in CtUnaryOperator which only consider the assignent-style unary operators.