Optimize copying during C++ and Rust data conversion.

[zhaohaidao]

Search before asking

• I searched in the issues and found nothing similar.

Description

Copying large blocks of data may cause performance bottlenecks. We should optimize the copying method if necessary.

Willingness to contribute

• I'm willing to submit a PR!

[Kelvinyu1117]

I would like to try that @zhaohaidao. I have took a look the PR #83. A rough idea is that The Datum should just hold the reference of the string_val and bytes_val if it is possible.

For string_val and bytes_val, we can use std::string_view and std::span (C++20) respectively.