We need to remove old mirror API: the _Reflectable protocol and all related underscored APIs.
1. All old mirrors in the standard library and SDK overlays should be migrated to the new CustomReflectable APIs. All mirrors that didn't have tests and are affected by this conversion should get tests.
2. dump() function should be ported to use the new mirrors.
3. The new mirror API should be migrated off the old _reflect() API. The issue is that _reflect() function and its implementation details depend on the old mirrors. We need to design new runtime entry points that make sense for the new mirrors implementation. This part requires writing a proposal, because the runtime API is going to be stable.
4. Remove old underscored mirrors. Remove old runtime entry points.