Make elastic_integer an alias

[johnmcfarlane]

The NumDigits in cnl::elastic_integer<NumDigits, Integer> may have been a mistake. Repurpose it as promote_integer<Integer> so that, e.g. operator+ doubles the width of the result.

New layout:

template<typename Rep>
promote_integer;

template<int Digits, typename Narrowest>
wide_integer;

template<int Digits, typename Narrowest>
using elastic_integer = promote_integer<wide_integer<Digits, Narrowest>>;

[elcojacobs]

I'm not sure I fully understand what you mean here, but it would be very useful for me if I can define a fixed_point<24,0, int32_t>, which does not saturate its underlying type.
When two of these are multiplied, I need a promoted type, but for addition, the type already has enough room to grow.

The elastic integer seems to provide exactly that: promoting to a bigger type only when it is needed. For addition, it only needs one extra integer bit and if and if there are unused digits left, no underlying type promotion is needed.

[johnmcfarlane]

The alias version of elastic_integer would do this. Because cnl::digits and cnl::set_digits would be used by promote_integer to decide the appropriate width of return types, and because wide_integer is bit-resolution, you'd get the same extra single bit when adding and double the bits when multiplying.

You can get your fixed-point type currently using fixed_point<elastic_integer<24>>. When they are added, you get fixed_point<elastic_integer<24>>. That wouldn't change if elastic_integer became an alias but wide_integer would now take on the job of keeping track of the exact number of bits.

[johnmcfarlane]

This appears to be captured/superseded by #864.