Auto merge of #28632 - alexcrichton:update-match-indices, r=Kimundi

This commit updates the `MatchIndices` and `RMatchIndices` iterators to follow
the same pattern as the `chars` and `char_indices` iterators. The `matches`
iterator currently yield `&str` elements, so the `MatchIndices` iterator now
yields the index of the match as well as the `&str` that matched (instead of
start/end indexes).

cc #27743

diff --cc src/libcore/str/mod.rs
Simple merge