Boost Review: Rationalize traits class usage to make the code easier to read.

[jzmaddock]

(1) Sometimes there are traits used, such as detail::bias_v<>, but other times the trait's functionality is replicated in-line. I find this more difficult to read, instead of using the trait. As an example, in the function detail::to_chars_impl(), we have this line:

constexpr auto max_fractional_value = std::is_same<TargetDecimalType, decimal32>::value || std::is_same<TargetDecimalType, decimal32_fast>::value ? TargetDecimalType{1, 7} : std::is_same<TargetDecimalType, decimal64>::value || std::is_same<TargetDecimalType, decimal64_fast>::value ? TargetDecimalType{1, 16} : TargetDecimalType{1, 34};

This line could easily be replaced with the following, which means exactly the same thing, but is easier to visually parse.

constexpr auto max_fractional_value = TargetDecimalType{1, bias_v<TargetDecimalType>};

I don't want this to sound like a nit-pick, but cases like this have made it more difficult for me to understand and work through the code.

(2) On the topic of traits, I think there should be some more traits defined, to remove usage of things like std::is_same<...>::value. This is another thing that has made it more difficult for me to read the code. Something along the lines of detail::is_fast<...> to denote whether you have a _fast type or not. For example, you could replace the following

std::enable_if_t<std::is_same<DecimalType, decimal32>::value || std::is_same<DecimalType, decimal64>::value || std::is_same<DecimalType, decimal128>::value, bool>

With the following

std::enable_if_t<!detail::is_fast<DecimalType>::value, bool>

As well, detail::bit_width<...> would help remove lines like std::is_same<T, decimal128>::value || std::is_same<T, decimal128_fast>::value, and turn them into detail::bit_width<...>::value == 128. These are just suggestions and don't need to be taken verbatim. Anything to remove duplication and make the code easier to understand.