Enable playground for crates that are available there

When a crate is available in https://play.rust-lang.org we could add the --playground-url argument so that its examples get the "Run" button enabled. The list of crates currently supported is at https://play.rust-lang.org/meta/crates (@shepmaster are you ok if we use this url? (with appropriate fallback in case it changes)).

One small complication is crates that have multiple versions available:

    {
      "name": "ansi_term",
      "version": "0.12.1",
      "id": "ansi_term"
    },
    {
      "name": "ansi_term",
      "version": "0.11.0",
      "id": "ansi_term_0_11_0"
    },

the playground puts older versions under a renamed extern, so we wouldn't want to link them. This probably means we just need to do a lookup based on the id == lib_name and then semver comparison that it's within the same semver compatible range (in either direction, since docs will generally be built for a newer version before it gets deployed to the playground).

The bigger complication is that we would be making static data linking to dynamic data, crates come and go from the playground, and it updates their versions all the time. I think it's probably still worth it, from what I've seen the playground crate list changes slowly enough that I think the upside of having the button when it works will outweigh the times it doesn't.

Start by tracing how docs.rs examples receive the --playground-url setting and how the "Run" button is enabled. Use the playground /meta/crates data, including crate IDs and versions, to define the compatible-version lookup and fallback behavior. Done means supported crates link to the playground without linking older renamed versions, while unavailable or changed crates remain safe.