Rust stable support for 1.27+

[tarcieri]

Rust 1.27 will be released momentarily. It includes a stable API for SIMD on x86/x64:

https://github.com/rust-lang/rust/blob/f8c8701b18b88ca3a96b2cf83f76a12fb1426e74/RELEASES.md#libraries

miscreant.rs (master) should be nearly ready to support stable. Here's a quick checklist:

• [x] Add stable to the list of Rust CI targets
• [x] Ensure Miscreant builds on Rust 1.27
• [x] Ensure documentation consistently reflects stable Rust support (and the needed RUSTFLAGS)

[spacekookie]

Currently it seems to fail to compile one of the dependencies because of a feature flag that's used:

error[E0554]: #![feature] may not be used on the stable release channel
 --> /home/spacekookie/.cargo/registry/src/github.com-1ecc6299db9ec823/clear_on_drop-0.2.3/src/lib.rs:2:34
  |
2 | #![cfg_attr(feature = "nightly", feature(asm, i128_type, specialization))]
  |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0554`.
error: Could not compile `clear_on_drop`.
warning: build failed, waiting for other jobs to finish...
error: build failed

Ah nvm it helps to remove a nightly flag from the dependency :wink:

[tarcieri]

181 fixed the code. Now we just need to do a pass on the docs.