Provide Rust API

[randomPoison]

Currently we primarily expose a C API that matches dav1d's API in order for rav1d to be a drop-in replacement for dav1d. However we also want rav1d to be usable in pure Rust projects, so we should also expose a Rust API.

For the most part the dav1d_* functions already call into equivalent rav1d_* Rust functions, e.g. dav1d_open simply wraps and calls into rav1d_open. But the Rust functions are not currently exported publicly (they're currently pub(crate)) and do not have any documentation.