o Minor bugfixes (Rust FFI):
    - Fix a minor memory leak which would happen whenever the C code would call
      the Rust implementation of protover_get_supported_protocols().  This was
      due to the C version returning a static string, whereas the Rust version
      newly allocated a CString to pass accross the FFI boundary.  Consequently,
      the C code was not expecting to need to free() what it was given.  Fixes
      bug 25127; bugfix on 0.3.2.1-alpha.