Significantly cleanup zerogc_derive

• e225f32: Mark binary_trees_parallel::Tree with collector_id(SimpleCollectorId)
• b337b2f: Add procedural unsafe_gc_impl! macro for manual impls (#22)
  • This siginificantly simplifies the implementation for stdlib types
• Implement NullTrace for PhantomData
• 010be99: Add unsafe attribute to skip generating destructor
• aee5449: Make #[zerogc(nop_trace) implicitly skip the Drop impl
• bbb8d0b: Add #[derive(NullTrace)] as sugar for #[zerogc(nop_trace)]
• ab5fcbb: Drop parking_lot's usage of 'nightly' feature
  • NOTE: The simple collector still depends on other nightly features..