Auto merge of #5597 - esamudera:slice_iter_next, r=flip1995

New lint: iter_next_slice

Hello, this is a work-in-progress PR for issue: https://github.com/rust-lang/rust-clippy/issues/5572

I have implemented lint to replace `iter().next()` for `slice[index..]` and `array` with `get(index)` and `get(0)` respectively. However since I made a lot of changes, I would like to request some feedback before continuing so that I could fix mistakes.

Thank you!

---

changelog: implement `iter_next_slice` lint and test, and modify `needless_continues`, `for_loop_over_options_result` UI tests since they have `iter().next()`

clippy_lints/src/lib.rs

@@ -666,6 +666,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &methods::INTO_ITER_ON_REF,
         &methods::ITERATOR_STEP_BY_ZERO,
         &methods::ITER_CLONED_COLLECT,
+        &methods::ITER_NEXT_SLICE,
         &methods::ITER_NTH,
         &methods::ITER_NTH_ZERO,
         &methods::ITER_SKIP_NEXT,
@@ -1310,6 +1311,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::INTO_ITER_ON_REF),
         LintId::of(&methods::ITERATOR_STEP_BY_ZERO),
         LintId::of(&methods::ITER_CLONED_COLLECT),
+        LintId::of(&methods::ITER_NEXT_SLICE),
         LintId::of(&methods::ITER_NTH),
         LintId::of(&methods::ITER_NTH_ZERO),
         LintId::of(&methods::ITER_SKIP_NEXT),
@@ -1491,6 +1493,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::CHARS_NEXT_CMP),
         LintId::of(&methods::INTO_ITER_ON_REF),
         LintId::of(&methods::ITER_CLONED_COLLECT),
+        LintId::of(&methods::ITER_NEXT_SLICE),
         LintId::of(&methods::ITER_NTH_ZERO),
         LintId::of(&methods::ITER_SKIP_NEXT),
         LintId::of(&methods::MANUAL_SATURATING_ARITHMETIC),