swift234

Index protocol extensions

- Remove free Swift functions for advance and distance and replace
  them with protocol extension methods:
  - advancedBy(n)
  - advancedBy(n, limit:)
  - distanceTo(end)
- Modernize the Index tests
  - Use StdlibUnittest
  - Test for custom implementation dispatch

Perf impact: No significant changes reported in the
Swift Performance Measurement Tool.

rdar://problem/22085119

Swift SVN r30958