Improve panic! messages in hex-literal

[thomascastleman]

Thanks for making the hex-literal crate! I was using it with an escaped hex string I'd copied from a C program, but ran into a panic because I'd accidentally left a semicolon after the strings I was calling hex!() on. It took me an embarrassing amount of time to discover the mistake, but I thought the panic messages might be able to provide a bit more info about what caused the failure.

This PR does only a few small things:

• I added a bit more info to the panic messages when you encounter a non-literal, invalid character, or odd number of hex characters, to give you context about what's causing the issue.
• I also added some internal documentation on some of the TokenTreeIter functions.
• Lastly, I added some tests for the core hex!() macro functionality in the tests/ directory (this was the only way I could find to set this up so the tests could invoke hex!()).

I didn't touch the changelog because I figure if this gets merged it's not a substantial enough change to warrant that. Thanks and I'd appreciate any feedback.

[tarcieri]

Looks good!

[newpavlov]

Thank you, looks good! I have only one small comment.

Also can you please update the changelog with an [UNRELEASED] section?