Use traits to control if collectors are singletons or thread safe

Now the zerogc-context crate can support singleton and multi-instance collectors in the same compilation. Likewise, it can also support thread-safe and thread-unsafe collectors in the same compilation. The simple implementation is still controlled by cfg-features but this is a step in the right direction.

• Implement Trace and GcSafe for String
  • I cant believe I missed this -_-
• [derive] Support ignoring parameters for #[derive(Trace)]
• [derive] Support deriving NullTrace via #[zerogc(nop_trace)] attribute