Add versioning info in-source and in git

[peter-moran]

For the sake of dependency management, it is really nice to have official release version numbers so that I can share with coworkers what version I am using.

I would suggest starting to git tag periodically with code changes so that those changes are versioned -- github has a nice way to do this with release docs that will show up on the project https://help.github.com/en/github/administering-a-repository/managing-releases-in-a-repository

Additionally, I would suggest adding something like the following to the source:

#define VERSION_STRING "v1.2.3"
#define VERSION_MAJOR 1
#define VERSION_MINOR 2
#define VERSION_PATCH 3

This would be really helpful! Thanks for the awesome project.

[tcbrindle]

Up until now I've been reluctant to do this because the design of span was still evolving. Now that C++20 has been finalised, this does indeed seem like a reasonable idea.

Once we meet the latest spec (see #32) I'll tag off a 1.0.0 release, and attempt to follow the semver rules after that.

[alejandro-colomar]

ping

[reinder]

For my project I log at startup all version info of 3rd party components that are used, would be great to add one for tcb::span too :)

Thanks for all the great work, nice piece of code, really helpful!