Consider using path-like type instead of string for arguments to functions

[AndrooFrowns]

In some functions such as save_to_xlsx or save_to_icalc, the functions take an argument of type &str and immediately convert to std::path::Path.

For convenience/clarity it might be worth taking either &Path as the argument type or <P: AsRef<Path> so that converting into the Path type is not required so often, and it's more clear which Strings represent paths rather than more general strings.

There is some discussion here about how to minimize the binary bloat of the monomorphized version.