support libbz2-rs-sys

[folkertdev]

Adds a feature flag that uses libbz2-rs-sys instead of bzip2-sys. The libbz2-rs-sys crate is a pure rust translation/implementation of stock bzip2. It is 100% compatible with stock bzip2.

Some notes:

• Cross-compilation (e.g. to wasm) should be much easier than with the C FFI
• Rust code is always linked statically (until we get a stable ABI, which is not close)
• When used as a rust crate, the rust global allocator is used for all memory allocation by default
• Our MSRV is pretty high right now. We can try to drop that a bit but e.g. let else is really convenient. The MSRV is unchanged with default settings.

I've not yet added any documentation (perhaps even the static feature flag deserves a mention somewhere). I've also not adjusted CI. Let me know what you want to see there (if anything).

[alexcrichton]

Thanks!

Also if you'd like I'd be happy to add you as a maintainer of this crate as well (or transfer ownership, I haven't worked on it in quite awhile at this point)

[folkertdev]

Cool! @trifectatechfoundation would be happy to adopt and maintain bzip2-rs.