lead(), lag(), shift()

[romainfrancois]

From #34

Not sure what to do with the ts case that the dplyr version dealt with specifically :

funs::lag(ts(1:10))
#>  [1] NA  1  2  3  4  5  6  7  8  9
dplyr::lag(ts(1:10))
#> Error: `x` must be a vector, not a ts object, do you want `stats::lag()`?

^{Created on 2021-06-01 by the reprex package (v2.0.0)}

Should this also include ilead() .... from #46 ?