Reduce .unwrap() in libvoikko.rs

[ronjakoi]

The functions in libvoikko.rs do a lot of stuff with ffi::{CStr, CString} and use .unwrap() while doing it. .unwrap() will panic on error, which can cause stack unwinding. Stack unwinding across the FFI boundary causes undefined behavior.

So, these results need to be actually checked and the results propagated in a controlled way. Maybe we need to introduce something like a StringError in the voikko module, unless we want to return something like Result<String, ffi::IntoStringError> across our API...