This patch covers addition between fixed point types and other fixed point types or integers, using the conversion rules described in 4.1.4 of N1169.

Usual arithmetic rules do not apply to binary operations when one of the operands is a fixed point type, and the result of the operation must be calculated with the full precision of the operands, so we should not perform any casting to a common type.

This patch does not include constant expression evaluation for addition of fixed point types. That will be addressed in another patch since I think this one is already big enough.