Fix cstr macro leak

[pnehrer]

The cstr macro uses CString::into_raw() to get the c-compatible string pointer.

However, per Rust doc, into_raw actually leaks the pointer if it isn't reclaimed and deallocated using from_raw.

This PR changes the macro and its usage to:

1. Create a CString
2. Keep it alive as we take its pointer (as_ptr)
3. Pass its as_ptr to the c functions

[abrown]

This is a good catch! Looks good to me.