Add shared group inclusion backtrace to example metadata.
Improve shared group backtrace in failure output.

- Show all shared group inclusion stack frames,
  not just the inner-most one.
- Make it work for all methods of including
  shared example groups. Previously, it only
  worked for the methods that generate a nested
  group.

Fixes #1209.
Merge pull request #1763 from rspec/shared-group-inclusion-backtrace

Improve shared group backtrace in failure output.