factor variances into a proper query

There are now two queries: crate and item. The crate one computes the
variance of all items in the crate; it is sort of an implementation
detail, and not meant to be used. The item one reads from the crate one,
synthesizing correct deps in lieu of the red-green algorithm.

At the same time, remove the `variance_computed` flag, which was a
horrible hack used to force invariance early on (e.g. when type-checking
constants). This is only needed because of trait applications, and
traits are always invariant anyway. Therefore, we now change to take
advantage of the query system:

- When asked to compute variances for a trait, just return a vector
  saying 'all invariant'.
- Remove the corresponding "inferreds" from traits, and tweak the
  constraint generation code to understand that traits are always
  inferred.

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