Improved build staging and control about plrust

[workingjubilee]

Hm. UI design is not trivially obvious here.

Major consideration: It may seem like this is a "validation" step and so it wants to be part of the "validator function", but is that the best idea? That predetermines the validator function call point: before the build is actually run. And the validator also is always called with no arguments, which makes it annoying to configure what the validator actually does in terms of checks aside from already-supplied methods. We want to have this as an option for catching a nefarious build.rs that might be executed, and we want to have an unambiguous path to doing so.

[workingjubilee]

Preferred UI design remains unclear but this got partially unblocked on the code level by the introduction of a specially-reserved "validation" stage in #128. A requirement that was revealed by that initial implementation is that this should not actually need to write the lib.rs to the filesystem twice for the built/audited source code, and that was satisfied in #143.