This attempts to centralize all the edition stuff in one place (the
`Edition` enum) so that adding a new edition or stabilizing one should
be relatively little work (and more importantly, we don't miss things).
Importantly, this changes `cargo new` to default to the latest stable.
It also changes the `cargo fix --edition-idiom` behavior to only apply
idioms for the *current* edition.
This commit stabilizes the `edition` key in `Cargo.toml`, both in the
`[package]` section and inside subtargets. Additionally the `cargo new` and
`cargo init` subcommands have been enhanced with a `--edition` flag to allow
explicitly specifying the edition to be generated.
This commit does not yet change the default edition that's generated.
Closes#5980
Previously build scripts were accidentally and unconditionally compiled with the
2015 edition, but they should instead use the edition of the `[package]` itself.
Closes#5860
