Replace string_tree with String for message buffering (#20)

[GG-O-BP]

Replaced the string_tree buffer with plain String in the chrome actor's process_port_message, as discussed in #20.

Ran some benchmarks and String came out faster in every scenario even repeated buffering where string_tree should theoretically have the advantage. The biggest difference was on large messages (~100KB) where String was about 3x faster.

The code also gets simpler since we no longer need string_tree in the hot path.

Closes #20

[GG-O-BP]

The benchmark module used for testing was excluded from this PR to keep it clean. Results are in #20. Happy to provide the module separately if needed.

[JonasGruenwald]

Thanks! This looks good to me, seems like the CI issue is unrelated to these changes.