Change parse assertion into an error log in release

[seanmonstar]

As I mentioned in #2534, I believe we should adjust that internal assertion to not panic in release mode. The purpose of that check is to ensure that httparse and http::HeaderName agree on what is a valid value. We can generally assume they will agree, but it's still possible for bugs, so that's why we check it. I propose the following action:

Write a new macro, maybe bug! (names are hard), that has the following behavior:

• When under cfg(debug_assertions), continue to panic with the value it does currently.
• When not, log a helpful message at the error! level, including that this is a bug and please report it.
• As part of the non-panicking variant, the user of the macro should include an error value to return.

The suggestion to use a new error variant seems like a good idea too. Perhaps Parse::Internal or Parse::Bug.

[bensadiku]

I've already started on #2544, if no-one else is already on it, I'll implement these changes there :)

[nox]

@bensadiku Thanks for your help, I've assigned the issue to you :)