Add s2_cell <-> integer64 support

[paleolimbot]

Closes #224.

library(s2)
(cells <- c(as_s2_cell(NA_character_), s2_cell_sentinel()))
#> <s2_cell[2]>
#> [1] NA               ffffffffffffffff
(int64s <- bit64::as.integer64(cells))
#> integer64
#> [1] <NA> -1
as_s2_cell(int64s)
#> <s2_cell[2]>
#> [1] NA               ffffffffffffffff

^{Created on 2023-04-19 with reprex v2.0.2}

[codecov-commenter]

Codecov Report

Merging #231 (283faa9) into main (90ae67a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 283faa9 differs from pull request most recent head 2c00abf. Consider uploading reports for the commit 2c00abf to get more accurate results

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@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
+ Coverage   94.02%   94.04%   +0.01%     
==========================================
  Files          46       46              
  Lines        3517     3524       +7     
==========================================
+ Hits         3307     3314       +7     
  Misses        210      210              

Impacted Files	Coverage Δ
R/s2-cell.R	100.00% <100.00%> (ø)

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