feat(r): Add support for bit64::integer64() conversions

[paleolimbot]

This PR implements support for bit64::integer64(). This vctr type is a mostly lossless representation of a 64-bit (signed) integer that is helpful for representing very large numbers (or uses like s2_cell() that just need a way to store 64 bits losslessly).

This type isn't part of the default conversions because it requires bit64, which has a few dependencies. It is also not the default because there are some limitations in R that cause that object type to behave unintuitively in certain places (e.g., when indexing a vector). Because convert_vector() always accepts a target vector type, users can (now) override the default behaviour with a safer option if that matters for their use case.

library(nanoarrow)
suppressPackageStartupMessages(library(bit64))

(vec <- as.integer64(c(-3:3, NA)))
#> integer64
#> [1] -3   -2   -1   0    1    2    3    <NA>
(array <- as_nanoarrow_array(vec))
#> <nanoarrow_array int64[8]>
#>  $ length    : int 8
#>  $ null_count: int 1
#>  $ offset    : int 0
#>  $ buffers   :List of 2
#>   ..$ :<nanoarrow_buffer validity<bool>[8][1 b]> `TRUE TRUE TRUE TRUE TRUE T...`
#>   ..$ :<nanoarrow_buffer data<int64>[8][64 b]> `-3.00e+00 -2.00e+00 -1.00e+0...`
#>  $ dictionary: NULL
#>  $ children  : list()
convert_array(array, integer64())
#> integer64
#> [1] -3   -2   -1   0    1    2    3    <NA>

^{Created on 2023-09-01 with reprex v2.0.2}

[codecov-commenter]

Codecov Report

Merging #293 (f68278f) into main (65bb552) will increase coverage by 0.00%.
The diff coverage is 90.80%.

@@           Coverage Diff            @@
##             main     #293    +/-   ##
========================================
  Coverage   86.89%   86.90%            
========================================
  Files          68       66     -2     
  Lines       10450    10179   -271     
========================================
- Hits         9081     8846   -235     
+ Misses       1369     1333    -36     

Files Changed	Coverage Δ
r/R/convert-array.R	92.10% <ø> (ø)
r/src/materialize_int64.h	88.46% <88.46%> (ø)
r/R/as-array.R	94.65% <92.85%> (-0.32%)	⬇️
r/R/schema.R	95.93% <100.00%> (+0.03%)	⬆️
r/src/convert.c	92.17% <100.00%> (+0.05%)	⬆️
r/src/convert_array.c	86.92% <100.00%> (+0.10%)	⬆️
r/src/materialize.c	80.74% <100.00%> (+0.14%)	⬆️

... and 4 files with indirect coverage changes

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