Add a compiler warning for increment/decrement on Nullable type

[ggleyzer]

Since Nullable is an Enum and therefore Sequential, it inherits nextValue() and prevValue() API and "++" and "--" ops. For example,

Int? i = Null;
i++;

successfully compiles and throws OutOfBounds exception.

Dima and Cam suggested to issue a compiler warning if those ops applied to Nullable type. The warning should say:

Operation is not permitted, because the type is Nullable, so the value may be Null.