Make a decision regarding allowed region names

[bencopl]

Currently it's hard to write a unit test for the region name as what's allowed depends entirely on the limitations (or seemingly lack of limitations) of std::string.

I've tried passing through super long region names, blank spaces, non-null-terminated strings, special characters etc. but it seems to be OK with all of them.

If we want some region to be not allowed, I think we will have to make assertions for it within the code or something similar that can easily be unit tested.

Reasons for not allowing certain things might be just be to make sure the output looks nice (e.g. isn't formatted weirdly because a region name is 100 characters, or has random gibberish due to non-null-terminated strings).