Remove explicit HC flag and auto-detect it instead

[patricksurry]

This mostly affects body> and improves see for HC words. It also removes a bunch of post-create rewriting with a new create_common entry point.

This leaves only four user-visible flags: AN (only used by exit in the builtins), NN (several builtins), and CO, IM. This should make it easier for users (esp beginners) who'll rarely need to think about any flags for new words.

The flexible header work will use three of the four free ones internally to manage the header structure.

[patricksurry]

Looking back at this, I'm not sure it's worth the cost of getting rid of the HC flag until we really need the bit for something else. Tempted to close this one and make a smaller PR that just has the SEE PFA cleanup. wdyt?

[SamCoVT]

That's fine with me. I think you've explored it enough to see what can be done, and now you're at the point where you can intelligently decide what should be done.

[patricksurry]

replaced by #124