fix(r): Warn for possibly out of range int64 -> double conversions

[paleolimbot]

It looks like I had left this as a TODO after my initial push to get most conversions implemented. The idea is to warn when converting very large int64/uint64 values to double. This an important case to warn for because double is the default conversion from int64 and values that won't (or might not) roundtrip back to int64 are important to warn about. See #293 implementing a safer conversion.

library(nanoarrow)

array <- as_nanoarrow_array(2^54, schema = na_int64())
convert_array(array, double())
#> Warning in convert_array.default(array, double()): 1 value(s) may have incurred
#> loss of precision in conversion to double()
#> [1] 1.80144e+16

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

[codecov-commenter]

Codecov Report

Merging #294 (d622ff4) into main (65bb552) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
- Coverage   86.89%   86.87%   -0.03%     
==========================================
  Files          68       65       -3     
  Lines       10450    10107     -343     
==========================================
- Hits         9081     8780     -301     
+ Misses       1369     1327      -42     

Files Changed	Coverage Δ
r/src/materialize_dbl.h	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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