Move arithmetic definitions into a submodule `FixedPointNumbers.FixedPointArithmetic`
cf. https://github.com/JuliaMath/FixedPointNumbers.jl/issues/142#issuecomment-699642981
I will move the definitions of checked_*/saturating_*/wrapping_* to CheckedArithmeticCore.jl and add the implementation for integer types there. (cf. https://github.com/JuliaMath/CheckedArithmetic.jl/issues/9)
In addition to @checked, @wrapping and @saturating macros will be provided by CheckedArithmetic.jl. Those macros should be available for FixedPoint as well.
Since we got the package extension mechanism, the new module should be something like FixedPointNumberCheckedArithmeticCoreExt.
See: https://github.com/JuliaMath/OverflowContexts.jl/pull/7