Add support for casting UFixed to SFixed and vice versa

[miracoli]

This fixes #20. If someone attempts to cast a negative SFixed to UFixed this will give the maximum integer for a specific UFixed minus the SFixed value. For example -1 will result to 255 and -1.5 will result to 254.5 etc.
If I am not mistaken, the behavior is therefore similar to that of integers.

[Pharap]

I am closing this because I have independently created a different solution (#93) to issue #20, which I happen to prefer because it is significantly simpler. Interestingly, it would seem that the implementation I arrived at is more or less functionally equivalent to the solution you created. Had I noticed this sooner, I could have potentially saved myself some testing in regards to the behaviour (i.e. with -1 becoming 255 and -1.5 becoming 254.5 et cetera).

Once again, I appreciate your attempts to contribute to the project (which I hopefully will be able to spend a bit more time on this year).