Potential for triggering debug assertion failure on Windows

[borrrden]

std::isalnum, std::isspace, and std::isdigit all have assertions when built with the debug C++ runtime that get violated when a value is not in the property range (0 - 255) but since the type used in this library's is_alnum, etc, is char the range is actually -127 - 127 meaning that any bytes above 127 will cause a debug assertion failure.