Don't set names on hop() results

[DavisVaughan]

Because the output size varies, so there is not a 1-1 mapping of input names to output size

slider::hop(
  c(x = 1, y = 2, z = 3, f = 4),
  .starts = c(-1, 3),
  .stops  = c(2, 6),
  ~.x
)
#> Error: The size of `names`, 4, must be the same as the size of `x`, 2.

^{Created on 2020-03-20 by the reprex package (v0.3.0)}

[DavisVaughan]

This propagates through to slide_period() as well