Re: plruby: rb_iterate symbol clash with libruby.so

I wrote:
> Yeah.  The long and short of this is that we're trampling on namespace
> that reasonably belongs to Ruby --- if they had some functions named
> "pg_something" and complained about a collision with libpq, would we
> change?  Nope.  So really we should rename these.

> Barring objections I'll go make this happen shortly.

Done.  I realized that the immediate problem, rb_iterate(), was only
added as of PG v10, which may explain why we hadn't heard complaints
about this till now.  So I've made the change only as far back as v10.
In principle we could change the rbtree code in 9.5/9.6 as well, but
I think that's more likely to create problems than fix any.

			regards, tom lane