Directly write to MutableBuffer in substring

[viirya]

Which issue does this PR close?

Closes #1422.

Rationale for this change

substring               time:   [30.604 ms 30.681 ms 30.767 ms]                   
                        change: [-64.758% -64.555% -64.358%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  6 (6.00%) high mild
  7 (7.00%) high severe

What changes are included in this PR?

Are there any user-facing changes?

[viirya]

cc @sunchao @alamb

[sunchao]

LGTM with one nit

[sunchao]

nit: "a random"

Maybe: "Creates a random (but fixed-seeded) array of strings with given size, null density and length"

[viirya]

Fixed.

[viirya]

Actually I copied from other functions here. 😄

[codecov-commenter]

Codecov Report

Merging #1423 (9de478b) into master (1fbe618) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1423      +/-   ##
==========================================
+ Coverage   83.13%   83.17%   +0.03%     
==========================================
  Files         182      182              
  Lines       53321    53439     +118     
==========================================
+ Hits        44330    44449     +119     
+ Misses       8991     8990       -1     

Impacted Files	Coverage Δ
arrow/src/compute/kernels/substring.rs	98.24% <100.00%> (-0.04%)	⬇️
arrow/src/util/bench_util.rs	95.45% <100.00%> (+0.21%)	⬆️
arrow/src/datatypes/datatype.rs	66.40% <0.00%> (-0.40%)	⬇️
arrow/src/array/ffi.rs	100.00% <0.00%> (ø)
arrow-flight/src/utils.rs	0.00% <0.00%> (ø)
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (ø)
...ng/src/flight_server_scenarios/integration_test.rs	0.00% <0.00%> (ø)
arrow/src/array/array_binary.rs	93.28% <0.00%> (+0.05%)	⬆️
arrow/src/array/equal/mod.rs	93.24% <0.00%> (+0.33%)	⬆️
arrow/src/ipc/reader.rs	87.79% <0.00%> (+0.94%)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1fbe618...9de478b. Read the comment docs.

[sunchao]

Merged, thanks!

[viirya]

Thanks @sunchao !

[alamb]

This is a nice change