Add cpp2::to_string(char const&) overload

[filipsajdak]

In the current implementation, cpp2::to_string will treat the char variable as an integral type and will print the integer value of char instead of the character.

main: () -> int = {
    c : char = 36;
    std::cout << "c = '(c)$'" << std::endl; // prints: c = '36'
}

after this change, it will print the character

main: () -> int = {
    c : char = 36;
    std::cout << "c = '(c)$'" << std::endl; // prints: c = '$'
}

[hsutter]

Looks good, thanks!