Correct oversights in Enum.slide/3

[s3cur3]

This fixes two issues I've discovered in v1.13-rc.0 (stuff that should have been in #11349 😬).

• Adds support for negative insertion indices:

    iex> Enum.slide([:a, :b, :c, :d, :e, :f], 1..2, -2)
    [:a, :d, :e, :b, :c, :f]
    iex> Enum.slide([:a, :b, :c, :d, :e, :f], -5..-3//1, -2)
    [:a, :e, :b, :c, :d, :f]
  

• Give a clear RuntimeError, rather than a baffling CondClauseError, when you ask for an insertion point that matches the last element of your range

    iex> XUtil.Enum.slide([:a, :b, :c, :d, :e, :f], 1..2, 2) 
    ** (RuntimeError) Insertion index for slide must be outside the range being moved (tried to insert 1..2 at 2)
  

[josevalim]

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