Rust edition 2018, no_std default, README and CI changes

[akhilles]

• extern crate ...; is deprecated.
• Cargo features should be used for "additive" functionality. So, it makes sense to remove the std feature.
• Modifying features requires version bump since it could be a breaking change.

[akhilles]

I get the need to consolidate breaking changes into a major version release. Can this PR be merged into a 2.0.0-preview working branch?

[mrhooray]

I'm good with working on top of master, optionally with 2.0.0-preview as version in cargo.toml. Gonna merge in next few days if no objections.

[akhilles]

@mrhooray can this be merged?

[mrhooray]

The build seems to fail on v1.31.0 https://travis-ci.org/mrhooray/crc-rs/builds/582183504

[akhilles]

Looks like int_to_from_bytes was stabilized in v1.32.0 and is needed for criterion. cargo bench should only run on nightly anyways. Will fix this.

[akhilles]

Turns out that criterion is compiled for cargo test as well as cargo bench. @mrhooray would you prefer to bump the MSRV to v1.32.0 or to keep using criterion v0.2?

[akhilles]

From criterion's compatibility policy:

Criterion.rs supports the last three stable minor releases of Rust. At time of writing, this means Rust 1.33 or later. Older versions may work, but are not tested or guaranteed.

Currently, the oldest version of Rust believed to work is 1.32. Future versions of Criterion.rs may break support for such old versions, and this will not be considered a breaking change. If you require Criterion.rs to work on old versions of Rust, you will need to stick to a specific patch version of Criterion.rs.

[mrhooray]

Happy with v1.32.0 unless you recommend otherwise.

[akhilles]

Bumped MSRV to v1.32.0 and squashed commits.