Improve efficiency of deserializing strings

[nnmm]

This addresses part of #214.

I recommend watching the excellent https://www.youtube.com/watch?v=BI_bHCGRgMY tutorial. I mostly followed the implementation of the standard library String type.

[esteve]

@nnmm cool! Looks like there's an issue with the generated Python files for rclrs_example_msgs, but it's not cause by this PR, might be a problem in rolling.

[nnmm]

@esteve yeah. Unfortunately the workaround suggested in ros-tooling/action-ros-ci#795 didn't seem to actually work around, so we'd have to wait for the next Rolling sync. Can we somehow make CI allow failure (for this specific check or entirely) until then?

[esteve]

@nnmm looks like that's a feature that users have requested (https://github.com/orgs/community/discussions/15452), I don't know of any other way to allow a job to fail. I think the best we can do now is disable the rolling CI job until the next sync.

[esteve]

@nnmm never mind, looks like continue-on-error might work in this case: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idcontinue-on-error

[esteve]

I've created #302 to allow the rolling job to fail, let's see if it works.

[nnmm]

@esteve Could you take another look now that CI is green?

[esteve]

@nnmm thanks!