Remove some unnecessary IdxSet methods
This replaces `IdxSet:: reset_to_empty` with `IdxSet:: clear`, and `IdxSet::elems`/`IdxSet::each_bit` with `IdxSet::iter`. Based on some [comments on #rustc](https://botbot.me/mozilla/rustc/2018-01-23/?msg=96063396).
r? @pnkfelix