OsString helpers #989
Description
Many of the shims @christianpoveda added recently need to read or write an OsString to/from the interpreter memory. Instead of copying that everywhere, we should have helpers that they can all call.
Moreover, right now these functions go through UTF-8. At least when running on a Unix host, we can do better by using OsStrExt, which gives us a raw byte buffer. We probably still need the UTF-8 code path as a fallback for cross-execution.