Added:

- `LookupSpans` implementation for `Layered` (#448)
- `SpanRef::from_root` to iterate over a span's parents from the root
  (#460)
- `Context::scope`, to iterate over the current context from the root
  (#460)
- `Context::lookup_current`, which returns a `SpanRef` to the current
  span's data (#460)

Changed:

- Lifetimes on some new `Context` methods to be less restrictive (#460)

Fixed:

- `Layer::downcast_ref` returning invalid references (#454)
- Compilation failure on 32-bit platforms (#462)
- Compilation failure with ANSI formatters (#438)

Signed-off-by: Eliza Weisman <eliza@buoyant.io>