support streaming read

Currently, a blocking function is provided by the library that reads from io::BufRead and writes to io::Write. This enforces the user of the library to read all contents into memory, or into a file.

Sometimes, however, it is only needed to traverse the data, but not have it all at once.

Such a thing could be achieved by having a function that, given io::Read, gives something that implements io::Read as well. This way, you can progressively read compressed or decompressed stream, while the library will internally read the underlying stream. This is how xz2 crate works, for example, see the function signature of xz2::read::XzDecoder::new. This also looks very flexible and intuitive as well: decompressor starts to act like a "pipe" (in unix terminology), rather than something that writes.

Support of it in lzma-rs would be very nice I think. Personally, I'm raising the issue because I wanted to try this library in rua https://github.com/vn971/rua Here I am using an intermediate layer of decompression for another function that accepts Read https://github.com/vn971/rua/blob/master/src/tar_check.rs#L26 (however, the underlying library xz2 is not pure Rust, but uses bindings)

Thoughts?