Safe-string handling in C/C++.

I'm fed-up with MSVC assertions in ctype functions (e.g. isdigit, isprint, isspace, etc.) when called with out-of-range (e.g. negative) values.

This problem only affects MSVC debug builds, but if you run them (like I do), these things are like little time bombs that can drive you crazy (knocking your board out of service).

The new macros names are bit more descriptive as well.

Created a contributing file to address what seems to be the most common reasons for rejected merge requests.