support for "table"

[romainfrancois]

originally tidyverse/dplyr#4963

vctrs::vec_c(table("tennis"))
#> Error: Can't find vctrs or base methods for concatenation.
#> vctrs methods must be implemented for class `table`.
#> See <https://vctrs.r-lib.org/articles/s3-vector.html>.

^{Created on 2020-03-10 by the reprex package (v0.3.0)}

[hadley]

@DavisVaughan could you please take a look at the minimal methods needed here?