Warnings by compose_tile_grid when sf is loaded

[16EAGLE]

Hi,

just an observation: while fixing broken moveVis code after the recent PROJ developments start to come into effect in spatial R packages on CRAN, I found that compose_tile_grid now throws the below warnings when it is loaded together with sf:

library(sf)
library(slippymath)
library(curl)

# get tile grid
bbox <- c(xmin = 8.897270, ymin = 47.721871, xmax = 8.999912, ymax = 47.773906)
tg <- bbox_to_tile_grid(bbox, max_tiles = 4)

# get image
url <- paste0("https://tile.openstreetmap.org/", tg$zoom, "/", tg$tiles[,1], "/", tg$tiles[,2], ".png")
files <- sapply(url, function(x){
  y <- tempfile(fileext = ".png")
  curl::curl_download(url = url, destfile = y)
  return(y)
})

# composing tile grid
r <- compose_tile_grid(tg, files)

# Warnings:
# 1: In `$.crs`(attr(bbox, "crs"), "proj4string") :
#   CRS uses proj4string, which is deprecated.
# 2: In `$.crs`(attr(bbox, "crs"), "proj4string") :
#   CRS uses proj4string, which is deprecated.
# 3: In `$.crs`(attr(bbox, "crs"), "proj4string") :
#   CRS uses proj4string, which is deprecated.
# 4: In `$.crs`(attr(bbox, "crs"), "proj4string") :
#   CRS uses proj4string, which is deprecated.

Session info

> version
#                _                           
# platform       x86_64-pc-linux-gnu         
# arch           x86_64                      
# os             linux-gnu                   
# system         x86_64, linux-gnu           
# status                                     
# major          3                           
# minor          6.3                         
# year           2020                        
# month          02                          
# day            29                          
# svn rev        77875                       
# language       R                           
# version.string R version 3.6.3 (2020-02-29)
# nickname       Holding the Windsock  

> packageVersion("slippymath")
# [1] ‘0.3.1’

No warnings when sf is not loaded. Thanks again for the great package!