Add BrotliDecompressStreamUninit for uninitialized buffer

[NobodyXu]

Having a safe API for MaybeUninit<u8> would be necessary for async-compression to wrap and reduce the performance penalty of initializing the buffer for all tokio::io::* traits implementation.

[CLAassistant]

All committers have signed the CLA.

[danielrh]

I want to ensure that we have a safe-only version of all APIs so someone can compile without any unsafe calls. Can you refactor this so that the unsafe and safe codes are duplicated and that way someone can simply link in the safe calls or the unsafe calls but don't need to go through an unsafe block if they want a safe interface.

[NobodyXu]

I could add a new type Buffer that:

enum Buffer<'a> {
    Uninit(&'a [MaybeUninit<u8>]),
    Init(&'a [u8]),
}

Then it'd be 100% free of unsafe. Does this look good to you?