don't allocate string when writing to csv

[ritchie46]

Writing to csv is pretty slow. Polars can read two datetime columns in 500 ms, whereas writing the same data to csv takes 33.6 seconds.

Upon looking at the code I saw we allocate a String for every value we format.

This PR makes sure we write immediately to the Vec<u8> buffer.

[codecov]

Codecov Report

Merging #935 (62d8263) into main (36b0824) will increase coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #935      +/-   ##
==========================================
+ Coverage   71.10%   71.12%   +0.01%     
==========================================
  Files         346      346              
  Lines       18951    18950       -1     
==========================================
+ Hits        13476    13479       +3     
+ Misses       5475     5471       -4     

Impacted Files	Coverage Δ
src/io/csv/write/serialize.rs	61.04% <88.88%> (+1.44%)	⬆️

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[ritchie46]

The clippy lints seem unrelated to these changes.

[jorgecarleitao]

oof, thanks a lot, @ritchie46 , really good stuff!