message: Remove SliceIndex for List
  Summary: It is not used, and I could not implement it with the following diff (some hard to understand problems with lifetimes).
diff: @@ -26,9 +26,8 @@ use std::{
  intrinsics::unlikely,
  marker::PhantomData,
  mem,
  - ops::{Deref, Index},
  + ops::Deref,
  slice,
  - slice::SliceIndex,
  };
  use gazebo::{
  @@ -172,15 +171,6 @@ impl<'v> List<'v> {
  }
  }
  -impl<'v, I: SliceIndex<[Value<'v>]>> Index<I> for List<'v> {
  - type Output = I::Output;
  -
  - #[inline]
  - fn index(&self, index: I) -> &Self::Output {
  - self.content.index(index)
  - }
  -}
  -
  impl<'v> Deref for ListRef<'v> {
  type Target = [Value<'v>];
  