Tracking issue for str::match_indices

[alexcrichton]

This is a tracking issue for the unstable str_match_indices feature in the standard library. This function should largely be good to go except for the note indicating that the returned iterator should probably yield different types of values to stay consistent with other methods.

cc @Kimundi

[aturon]

Nominating for 1.5 FCP discussion.

[alexcrichton]

This issue is now entering its cycle-long FCP for stabilization in 1.5

(assuming the signature change)

[alexcrichton]

The libs team discussed this during triage today and the decision was to stabilize.