fixes 'std::ptrdiff_t' to 'std::size_t' casting error and suppresses `-Wuseless-cast` warning for gcc and clang

[RalphSteinhagen]

Thanks for the great library! :+1:

We use this in our project, using compiler warnings and static-code analysis extensively. I didn't want to suppress all magic_enum warnings on the project level, but I thought of contributing some of the easy fixes:

fixes 'std::ptrdiff_t' to 'std::size_t' casting error

Addresses this compiler warning:

[..]/magic_enum_utility.hpp:101:31: warning: conversion to ‘std::size_t’ {aka ‘long unsigned int’} from ‘std::ptrdiff_t’ {aka ‘long int’} may change the sign of the result [-Wsign-conversion]
  101 |       return enum_value<D, S>(index);
      |                               ^~~~~

suppresses -Wuseless-cast for static_cast('\0')

Needed in case 'char_type' is 'char' (common on Linux but rare on Windows?)

[..]/magic_enum.hpp:275:114: warning: useless cast to type ‘using magic_enum::char_type = using std::basic_string_view<char>::value_type = char’ {aka ‘char’} [-Wuseless-cast]
  275 |   constexpr static_str(string_view str, std::integer_sequence<std::uint16_t, I...>) noexcept : chars_{str[I]..., static_cast<char_type>('\0')} {}
      |                                                                                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hope this helps and meets your coding standards.

Question aside: would you be willing to introduce compiler warning flags and static code analysis tools?

[Neargye]

Thanks!