Lexical cast validation for hffix::message_reader

[jamesdbrock]

It would be nice if the hffix::details::atoi and hffix::details::atou functions verified that the ASCII which they are converting to a number is in fact digits 0-9.

But how to do that elegantly?

Don't want to complicate the return type with std::optional, that would require ugliness and C++17.

Don't want to throw an exception, the library design is that we only throw exceptions for mistakes in API usage, not for some garbage that we got over the network.