Replace strval() function by using direct type casting to (string)

[mhauri]

Description

Direct type casting is up to 6x faster than using casting functions like strval(). So this pull request replaces the strval() functions by direct type casting to (string).

Similar PRs:

#16422 Replace intval() function by using direct type casting to (int) where no default value is needed
#16848 Replace floatval() function by using direct type casting to (float)

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @mhauri. Thank you for your contribution
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[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-2644 has been created to process this Pull Request

[magento-engcom-team]

Hi @mhauri. Thank you for your contribution.
We will aim to release these changes as part of 2.2.7.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.