Forbid creating IdStrings and incrementing autoidx during multithreaded phases, and add dynamic checks for that

We could make it safe to increment autoidx during multithreaded passes, but that's
actually undesirable because it would lead to nondeterminism. If/when we need new
IDs during parallel passes, we'll have to figure out how to allocate them in a
deterministic way, and that will depend on the details of what the pass does.
So don't try to tackle that now.

kernel/rtlil.cc

@@ -98,7 +98,7 @@ int RTLIL::IdString::really_insert(std::string_view p, std::unordered_map<std::s
 				return -*p_autoidx;
 			// Ensure NEW_ID/NEW_ID_SUFFIX will not create collisions with the ID
 			// we're about to create.
-			autoidx = std::max(autoidx, *p_autoidx + 1);
+			autoidx.ensure_at_least(*p_autoidx + 1);
 		}
 	}