Detect field assignment to zero-value as sanitization

[PurelyApplied]

Consider the following:

type Source struct {
  secret string // `levee:"source"`
}

func Bar(s Source) {
  s.secret = ""
  Sink(s)  // This should not emit a Diagnostic.
}

Explicitly setting to zero-values all fields containing source data should be recognized as sanitization.

[PurelyApplied]

Optionally, a user may wish to provide sanctioned non-zero values, e.g. s.secret = "REDACTED".

[PurelyApplied]

Cross-referencing #97, a wrapping type should be able to likewise sanitize via either of Container.content = nil or Container.content.secret = "".

[mlevesquedion]

I think we could handle some of these cases without too much headache, but in general this looks like it's going to be very tricky.

If we do a good job with it though, eventually we may be able to identify sanitizers automatically.