Set the signess of a variable in declaration if it is always used with a sign.

[mchalupa]

If a variable is cast to e.g. int everywhere in the program, we may declare it int instead of unsigned int (and the casts visitor then removes useless casts). The question is whether we can easily track the casts of the variable.

[tomsik68]

Currently, it's not possible to easily distinguish whether a Value is a variable or a constant. It'd be wise to work on that before fixing this.