FIX: Correct arithmetic operations

Fixis issues found in the discussion of #240:
Arithmetic operations are now done in their native datatype and only
casted if an overflow happens. This leads to better precision in results.
Also, hex strings are now allowed to have a sign (+ or -) if (and only if) they are
used in arithmetic operations.
This line triggers an overflow for `-0x8000000000000000`.