move bench logic into internal package

[vroldanbet]

So it makes Goland IDE able to compile the module correctly. 🙏🏻 feel free to reject this PR if these tool-specific workarounds shouldn't be made into the repository, I can totally understand it.

It's unfortunate that somehow Goland trips with Go code that lives in the main package, but thought it may be acceptable to the mantainer/s to move logic to an internal package as the pattern is already used in this repo and is effectively a noop.

[cschleiden]

Uh, what's the issue with Goland here?

[vroldanbet]

Nevermind, after some tinkering with the IDE, I managed to find a way for it to compile!