Stabilize on a rustc version

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At some point, explicitly choose a minimum rustc version with which
this library must be built. After so choosing, I could still use newer
rustc features, but only gated behind cfg flags.

To generate the proper cfg flags, see:

• https://docs.rs/rustc_version
• https://docs.rs/foreman
• http://doc.crates.io/build-script.html#outputs-of-the-build-script

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I've chosen rustc 1.22.1, the last release of 2017, as the minimum rustc version for the foreseeable future.

(Edit, 2018-09-01 UTC: I would think it irresponsible to support anything older than 1.22.)

Now I just need to document this.

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To escape a jobserver bug that was rendering me incapable of running cargo publish (see also: rust-lang/rust#46981), I'm upgrading to rustc 1.24.1 in commit 3bcb5e4.

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And now I'm upgrading to rustc 1.26.0, in commit 1d8d6ad.

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For bug fixes, I'm upgrading to rustc 1.26.2, in commit eeb3b1f.

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I likely will upgrade to Rust 1.29.1, for the security fix, with that becoming a general rule: a security fix in the standard library being considered justification for raising the minimum supported Rust version.

Incidentally, I think this also brings me stable Clippy, so that's nice (if it's true).

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I have upgraded to Rust 1.29.1, as predicted above, in commit f7ce4a7. This does seem to bring me stable Clippy.