Testing and documentation for arithmetic overflow

[saxbophone]

Arithmetic overflow can occur in the following scenarios:

• Addition
• Subtraction
• Multiplication
• Division
• Increment
• Decrement
• Creating PSXFixed from a double or from_integer() where the input is outside the bounds of the fixed-point type

Currently, the behaviour of overflowing operations is unspecified. It should be specified, tested for, and documented, regardless of whether we decide to catch it or not.