Make it possible to avoid the rustc_version and semver build dependencies

[briansmith]

Hi, I'd like to use cast.rs but I'd also like to minimize my dependency tree. I noticed that rustc_version seems to be used primarily to decide whether to support 128-bit types. But, 128-bit types have been in stable Rust for years. I think it would be a better trade-off to break compatibility with the ancient versions of Rust with a new version that eliminates this conditional check. I would be happy to submit a PR that does this if you agree this makes sense.

[japaric]

Hi, I think that sounds reasonable. I'd be OK with doing a 0.3.0 release that drops the rustc-version build dep, removes the x128 Cargo feature and removes the MSRV policy text from the docs. Feel free to submit a PR that does any part of that and I'll follow up with a release. Thanks for bringing this up!

[briansmith]

PR #37 fixes this.

[japaric]

done in #37