Copy token string when using FeatCtx::tokstr()

[iterumllc]

Description

I had thought that std::string objects retrieved from the parser tree are valid for the lifetime of the tree but there seems to be some other smart-pointer stuff going on. In most cases there is no difference (because strings are either used in the function where they're retrieved or passed "down") but FeatCtx::tokstr() needs to be changed to store the string.

I also changed the "reference token" for feature blocks to the start tag so that the error message makes sense. (That would have been a better choice to begin with anyway.)

Checklist:

• I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[kaydeearts]

👍 Thanks for this quick fix!