Make rclrs panic-free as far as possible

[nnmm]

This is achieved by making CString errors a part of RclrsError. This was made possible by #183, which made the error struct an enum.

Example error output:

Error: Could not convert string 'minimal_subscriber' to CString

Caused by:
   nul byte found in provided data at position: 13

The nul byte is not rendered in the output, as you can see.

[nnmm]

Note for reviewers: This does not increase the size of the RclrsError struct, since the other variants are larger.

The map_err() is of course a little verbose, but users should never need to do CString conversions anyway, so the API is clean. But still, if it's too ugly, we could remove the string member from this enum variant and impl From<NulError> for RclrsError.

[jhdcs]

These look to be fairly simple and reasonable changes to me.

[nnmm]

Thanks @jhdcs!