feat: impl std::io::Write for SliceDeque<u8>

[ErichDonGubler]

I haven't seen any discussion about this on this repo -- I've tried to do my homework, but I realize that this PR is out of the blue and hope that it's welcome. :) It should be a small commitment to review this, though, since the diff is small!

I've run into a case supporting some projects for vivint-smarthome where having std::io::Write would be particularly helpful for interoperation with compression crates like flate2.

This particular implementation comes straight from std's implementation for Vec<u8>. That means that the correctness of the implementation will just depend on SliceDeque::{extend_from_slice,reserve} having the same semantics as their counterparts in Vec<u8>, which I believe is the case.

[ErichDonGubler]

Hmm, there are some failures in CI that are concerning to me as a user but I don't think have to do with this PR, since they also happen on master in Travis. In no particular order:

• Not having an unstable feature (which seems to still be used in src/) causes failures on just about everything. Adding unstable = [] to [features] in Cargo.toml fixes over half of the builds for individual platforms, which I have added as a commit here. I can break it out though if you prefer.
• The next-largest number of issues comes from having clippy fail, looks like from new lint(s) that came with 1.41. Other builds are running Clippy despite having a dedicated job?
• Various other failures, haven't taken the time to enumerate them yet.

[ErichDonGubler]

@gnzlbg, ping!

[zyansheep]

This would indeed be nice to have, (I need it too!) however, I think the owner of this repo is not active on github anymore... does anyone know any alternatives to this crate?

[Boscop]

@zyansheep Yes, this fork: https://github.com/LiquidityC/slice_ring_buffer