Support buffer of [MaybeUninit<u8>]

[NobodyXu]

We should 100% support that, it would enable our users to pass uninitialised buffer for best performance, and our tokio impl in async-compression will be much, much faster after this.

We would start by redesigning our PartialBuffer API, if we want to keep backwards compatibility we can introduce a new trait Buffer so that user can still pass [u8].

But if we decided not to, then we can redesign PartialBuffer to support it, while also removing the generics for #385

The only sad thing is that we might need to cut a major release of async-compression, because it re-exports core and codecs

[Nemo157]

My plan long ago was to wait for std::io::BorrowedBuf and change PartialBuffer to be based around that, I'd kind of hoped that would have happened by now. IIRC none of the underlying compression libraries had support for uninit buffers anyway so even if Codec supported it all implementations would have to use ensure_init to get initialized buffers to pass down.

[NobodyXu]

Took a look at the tracking issue, it looks like there's no plan to stablise it soon

It's unfortunate that the safe rust API exposed by these crates does not support [MaybeUninit<u8>], given that many uses C API underlying (including rust implementation of libz-rs, libbzip2-rs which exposes C API), I think it won't be too difficult to have that.

Waiting for the tracking issue is going to be the easiest/simplest solution though

[NobodyXu]

Opened PRs to add [MaybeUninit<u8>] support:

• bzip2 Add MaybeUninit<u8> (de)compress API trifectatechfoundation/bzip2-rs#147
• deflate/flate/gzip/zlib Add (de)compress_uninit that accepts &[MaybeUninit<u8>] rust-lang/flate2-rs#502 released in 1.1.3
• lzma/xz/xz2 Add Stream::process_uninit to accept &[MaybeUninit<u8>] output Portable-Network-Archive/liblzma-rs#193 released in 0.4.5
• lz4 external C API, probably already supports uninitialized (safe to assume C API never reads from uninitialized buffer, since that'd be UB)
• zstd already exposes such functionality through WriteBuf, we just need to implement it
• deflate64 Add InflaterManaged::inflate_uninit for uninitialized buffer anatawa12/deflate64-rs#34 released in 0.1.10
• brotli-decompressor Add BrotliDecompressStreamUninit for uninitialized buffer dropbox/rust-brotli-decompressor#41
• brotli in progress

[NobodyXu]

I realize that we can support this without breaking existing APIs, just adding new methods to De/Encode traits