Non-optimal code generation for narrow<long long int>(char)

[HFriberg]

A quick test on Godbolt showed that the latest ICC compiler (v19.0.1) does not recognize that char can always be converted to long long int without loss of precision with the current implementation of narrow:
https://godbolt.org/z/73zPhG

I suggest wrapping the current implementation of narrow in a compile-time check on types, to see if there is any reason to check the values. Ideas:

1. One suggestion from stackoverflow I haven't tested is this:

if constexpr( !std::is_same<T, typename std::common_type<T,U>::type>::value )

2. Another suggestion is to use the is_narrowing_conversion definition from https://riccibruno.github.io/2018/04/27/c++_is_narrowing_conversion_type_trait/.