Overhaul `config::features`.

The new way of doing things:
- Avoids some code duplication.
- Distinguishes the `crate_edition` (which comes from `--edition`) and
  the `features_edition` (which combines `--edition` along with any
  `rustc_20XX_preview` features), which is useful.
- Has a simpler initial loop, one that just looks for
  `rustc_20XX_preview` features in order to compute `features_edition`.
- Creates a fallible alternative to `Features::enabled`, which is
  useful.

It's not easy to see how exactly the old and new code are equivalent,
but it's reassuring to know that the test coverage is quite good for
this stuff.

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