Fix rb_interned_str_* functions to not assume static strings

[casperisfine]

Followup: #3586

When passed a fake_str, register_fstring would create new strings with str_new_static. That's not what was expected, and answer almost no use cases.

[XrXr]

It makes sense to me that these functions should work on buffers that are not C string literals.

In that case, we are creating some new APIs instead of merely exposing some previously internal ones without modification, meaning the title of the Redmine ticket is kind of a misnomer. I bring this up to make sure that everyone is on the same page about what these new functions are supposed to do. I didn't follow the discussion for the ticket.

Code wise, I would prefer passing an additional parameter to fstr_update_callback instead of copy pasting and modifying it.

[casperisfine]

Code wise, I would prefer passing an additional parameter to fstr_update_callback instead of copy pasting and modifying it.

Yes that a good point. I'll try to clean this up.

[casperisfine]

@ko1 I'm sorry to bother you again with this but could you have a look? Without this PR I don't think rb_interned_str* is usable.

[ko1]

Thank you I'm asking @nobu

[casperisfine]

Thanks @nobu !

[casperisfine]

@ko1 is this good for you?

[ko1]

Ah, I have no opinion, thank you so much to provide your efforts!

[casperisfine]

Thanks!