Add support for vctrs::vec_cast

[colearendt]

In many cases, it would be helpful to have the ability to cast integers, etc. to fs_bytes. For instance:

library(experiment)

vctrs::vec_cast(as.integer(c(2L, 4L)), fs::as_fs_bytes(1L))
#> Warning: The `vec_cast.fs_bytes` function is experimental and subject to change without warning in a future release
#> This warning is displayed once per session.
#> 2 4

^{Created on 2020-04-15 by the reprex package (v0.3.0)}

This required the following, and will require a Suggests dependency on vctrs:

#' @export
vec_cast.fs_bytes.integer <- function(x, to, ...) {
  warn_experimental("vec_cast.fs_bytes")
  fs::as_fs_bytes(x)
}

#' @export
vec_cast.fs_bytes.default <- function(x, to, ...) {
  vctrs::vec_default_cast(x = x, to = to)
}

#' @export
vec_cast.fs_bytes <- function(x, to, ...) {
  warn_experimental("vec_cast.fs_bytes")
  UseMethod("vec_cast.fs_bytes")
}