incr.comp.: Some preparatory work for caching more query results.
This PR
* adds and updates some encoding/decoding routines for various query result types so they can be cached later, and
* adds missing `[input]` annotations for a few `DepNode` variants.
The situation around having to explicitly mark dep-nodes/queries as inputs is not really satisfactory. I hope we can find a way of making this more fool-proof in the future.
r? @nikomatsakis
4fa202d23b