Add integer typecheck guard to Series.shift

[blutack]

Add an integer typecheck guard to Series.shift, as discussed in issue #523.

This improves the error returned when calling Series.shift with a float or some other invalid datatype.

Before:

iex> Explorer.Series.shift(s, 1.0)
** (ArgumentError) argument error
    (explorer 0.6.0-dev) Explorer.PolarsBackend.Native.s_shift(#Explorer.PolarsBackend.Series<
  shape: (5,)
  Series: '' [i64]
  [
        1
        2
        3
        4
        5
  ]
>, 1.0)
    (explorer 0.6.0-dev) lib/explorer/polars_backend/shared.ex:23: Explorer.PolarsBackend.Shared.apply_series/3
    iex:3: (file)

After:

iex> Explorer.Series.shift(s, 1.0)
** (ArgumentError) Explorer.Series.shift/2 not implemented for dtype 1.0. Valid dtypes are [:integer]
    (explorer 0.6.0-dev) lib/explorer/series.ex:3622: Explorer.Series.dtype_error/3
    iex:3: (file)

[blutack]

Thank you, I noticed that dtype_error shouldn't be used outside of Series arguments and changed the default function to raise a formatted error directly, but just removing the unguarded function is more sensible. Please back the second commit out if you prefer to stick with that!

[josevalim]

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