String concat performance improve

[stdex]

Based on issue:
https://pear.php.net/bugs/bug.php?id=18866

Fix performance string concat

php 7.4, write xls by 5 cell in each row

rows :: seconds

[now]
10000 :: 0.4
20000 :: 1.1
30000 :: 2.1
40000 :: 9.9
50000 :: 25.0
65534 :: 46.6

[after fix]
10000 :: 0.2
20000 :: 0.4
30000 :: 0.6
40000 :: 0.8
50000 :: 1.0
65534 :: 1.4

[sanmai]

Thank you.

[sanmai]

Would you mind sharing the benchmark you used?

[stdex]

Would you mind sharing the benchmark you used?

Yes. I share benchmark here: https://github.com/stdex/xls-benchmarks
Some results available in Travis CI build: https://travis-ci.com/github/stdex/xls-benchmarks/builds/175971157

You also may notice me, that the results are slightly different from the ones I presented initially in this MR.

Travis:
------------------------------------------------------------------------------------------------------------
| Package                       | Version    | Test       | Avg Duration (MS) | Avg Peak memory usage (MB) |
------------------------------------------------------------------------------------------------------------
| pear/spreadsheet_excel_writer | dev-master | writerTest | 7937              | 19                         |
------------------------------------------------------------------------------------------------------------

Local:
------------------------------------------------------------------------------------------------------------
| Package                       | Version    | Test       | Avg Duration (MS) | Avg Peak memory usage (MB) |
------------------------------------------------------------------------------------------------------------
| pear/spreadsheet_excel_writer | dev-master | writerTest | 1424.62           | 16                         |
------------------------------------------------------------------------------------------------------------

Those the absolute results are highly dependent on the platform the script is run on.