This is better than using unsafe blocks inside the functions, as that
tells the compiler that the unsafeness won't leak out of the block,
which isn't true in this case as we're dealing with another unsafe
language.
The tts_speak() function has an additional parameter that, if not NULL,
will be filled with a pointer to an UtteranceId. If this is specified,
the caller must also call tts_free_utterance() to deallocate the
UtteranceId when they're done with it.
* tts_default() allocates a new TTS struct via it's default() constructor, returning a pointer to it or NULL on error.
* tts_free(tts) destroys the TTS pointed to by tts. If tts is NULL, this
function does nothing.
Any errors reported will cause the C API functions to return an error
value (NULL or -1). The caller can then use:
* const char* tts_get_error() to get a pointer to a string describing
the error
* void tts_clear_error() to deallocate any currently stored error
message.
mcb2003