feat(r): Improve printing and conversion of buffers

[paleolimbot]

After #207, buffer data types are readily available. Before, the R bindings did most of this on their own to get debug buffer printing. This PR exploits the new feature to improve debug output and adds the ability to convert buffers into R vectors along the way.

Before this PR:

library(nanoarrow)
as_nanoarrow_array(c(NA, stringr::words))
#> <nanoarrow_array string[981]>
#>  $ length    : int 981
#>  $ null_count: int 1
#>  $ offset    : int 0
#>  $ buffers   :List of 3
#>   ..$ :<nanoarrow_buffer_validity[123 b] at 0x13b71d8d0>
#>   ..$ :<nanoarrow_buffer_data_offset32[3928 b] at 0x13d87b600>
#>   ..$ :<nanoarrow_buffer_data_utf8[5126 b] at 0x13d87c600>
#>  $ dictionary: NULL
#>  $ children  : list()

^{Created on 2023-05-30 with reprex v2.0.2}

After this PR:

library(nanoarrow)
as_nanoarrow_array(c(NA, stringr::words))
#> <nanoarrow_array string[981]>
#>  $ length    : int 981
#>  $ null_count: int 1
#>  $ offset    : int 0
#>  $ buffers   :List of 3
#>   ..$ :<nanoarrow_buffer validity<bool>[984][123 b]> `FALSE TRUE TRUE TRUE T...`
#>   ..$ :<nanoarrow_buffer data_offset<int32>[982][3928 b]> `0 0 1 5 10 18 24 ...`
#>   ..$ :<nanoarrow_buffer data<string>[5126 b]> `aableaboutabsoluteacceptacco...`
#>  $ dictionary: NULL
#>  $ children  : list()

^{Created on 2023-05-30 with reprex v2.0.2}

[codecov-commenter]

Codecov Report

Merging #208 (b022bb3) into main (1e6ca7e) will decrease coverage by 0.10%.
The diff coverage is 81.16%.

@@            Coverage Diff             @@
##             main     #208      +/-   ##
==========================================
- Coverage   87.93%   87.84%   -0.10%     
==========================================
  Files          60       60              
  Lines        9164     9211      +47     
==========================================
+ Hits         8058     8091      +33     
- Misses       1106     1120      +14     

Impacted Files	Coverage Δ
r/src/init.c	100.00% <ø> (ø)
r/src/buffer.c	82.53% <78.68%> (-4.42%)	⬇️
r/R/buffer.R	76.28% <78.94%> (+2.21%)	⬆️
r/R/infer-ptype.R	100.00% <100.00%> (ø)
r/src/array.c	95.59% <100.00%> (+3.43%)	⬆️
r/src/buffer.h	94.59% <100.00%> (+1.26%)	⬆️

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