Add cargo downgrade or cargo update --oldest

[DanielKeep]

Currently, Cargo will try to use the newest version of dependencies when you invoke cargo update or just build for the first time. This is a pain when you're trying to write a package, as it means the version of dependencies listed in the manifest doesn't necessarily match the version of dependencies that are being tested/developed against.

As such, it would be very convenient if there was a shorthand command or switch that downgraded all dependencies of the current package to the earliest possible versions. For bonus points, it should signal an error if the version specified by a ^ (and possibly >=) constraint is not selectable (i.e. "you say I should be able to select plimsor="1.2.6", but I can't actually do so). Double bonus points for some persistent per-package option that makes this the default and/or prevents implicitly updating dependencies with some explicit override.

This should help keep package authors honest about what dependency versions their code actually works with.

[carols10cents]

I'm going to close this as a duplicate of #4100, which is part of rust-lang/rfcs#1977 :)