C-style API

[Macoron]

Recently, I was working on porting Piper to Unity3d. Unfortunately, current piper-phonemize API can't be marshalled directly in C#, because of std classes like vector or map.

I rewrote it to simpler "C-style" API here. The biggest changes are:

1. No usage of std C++ classes, only structs and pointers
2. Storing phonemize internal state and last result with init and destroy functions (to avoid allocating or deallocating memory in managed code)

I believe, this might be useful for other devs creating bindings for managed languages that doesn't support C++ classes. Would you be interested in PR with these changes? Should it be part of piper-phonemize or main piper repo?

[synesthesiam]

Sure, I'd be interested in a PR! Thanks 🙂