Move arithmetic definitions into a submodule `FixedPointNumbers.FixedPointArithmetic`

[kimikage]

cf. #142 (comment)

I will move the definitions of checked_*/saturating_*/wrapping_* to CheckedArithmeticCore.jl and add the implementation for integer types there. (cf. JuliaMath/CheckedArithmetic.jl#9)

In addition to @checked, @wrapping and @saturating macros will be provided by CheckedArithmetic.jl. Those macros should be available for FixedPoint as well.

[kimikage]

Since we got the package extension mechanism, the new module should be something like FixedPointNumberCheckedArithmeticCoreExt.

[kimikage]

See: JuliaMath/OverflowContexts.jl#7