Move utilities that don't need to be implemented twice to a separate file written in F*

Some of the functions in `FStar_Utils.ml` and `utils.fs` could be implemented a single time in F*. It would be nice to reduce the duplication and move them to an F* file — maybe something like `src/MoreUtils.fs`?

Compare the utilities in FStar_Utils.ml and utils.fs to identify functions that can be implemented once in F*. Review the proposed src/MoreUtils.fs location and determine the migration scope. Done means the selected utilities have a shared F* implementation without the current duplication.

Written by the indexing model from the issue text.