Rename `schema::Object.builtin` to `is_builtin`

We’re using an is_ prefix pretty consistently for bool flags. @vpetrovykh please fix this one and take a close look if we have other inconsistencies.

[vpetrovykh]

To clarify what I mean by “my sense of formalism is in strong opposition of actual natural language use” I mean that consistent is_ prefixes for bool flags seem appealing to me, personally. However, having them as part of the names makes it awkward to write actual English sentences like “If the property in question is is_required …” and forces re-wording like “If the property has an is_required flag set to true …”. In spoken English it’s not a big deal, but in a written manual/example it becomes a bit awkward.

So 1 IF link.is_required ELSE 0 may look good in code, but it’s awkward in docs. I think I just made a case against is_ prefix…

[elprans]

sn’t that exactly the distinction between all of shcema and user-introspectible schema that @elprans talked about?

That’s correct.