Rangechange the log message truncation limit.
Make supertrait methods callable on object types.

This requires changes to method search and to codegen. We now emit a
vtable for objects that includes methods from all supertraits.
Closes #4100.

Also, actually populate the cache for vtables, and also key it by type
so that it actually works.
auto merge of #8519 : msullivan/rust/objects, r=catamorphism

r?