Clean up / rewrite resolve

[marijnh]

Resolve has organically grown into a very complicated, rather slow, probably partially redundant tangle. When a decision is made on #1893 , some work should be done on cleaning it up.

[catamorphism]

labelled as postponed since it depends on another bug, and assigning to you, @marijnh , since you seem like the most qualified person to do it.

[brson]

I should mention that rustdoc uses resolve to locate reexports and needs it to be tolerant of errors.

[catamorphism]

We agreed that I will finish this, in Marijn's absence.

[catamorphism]

Reassigned to @pcwalton to reflect reality...

[catamorphism]

This is done; if any subsidiary issues show up, file separate bugs.