process trait/impl items directly from the visitor callback
The current setup processes impl/trait items while visiting
the impl/trait. This means we basically have this setup:
<Lots> -> TypeckItemBody(Impl) -> Tables(ImplItem{0,1,2,3})
But this was largely an artifact of the older code. By moving the
processing of items into method dedicated for their use, we produce this
setup:
<Little> -> TypeckItemBody(ImplItem0) -> Tables(ImplItem0)
...
<Little> -> TypeckItemBody(ImplItem3) -> Tables(ImplItem3)
r? @michaelwoerister
Also, we might consider removing the `TypeckItemBody` node altogether and just using `Tables` as the task. `Tables` is its primary output, I imagine? That would reduce size of dep-graph somewhat.
cc @eddyb -- perhaps this pattern applies elsewhere?
2f0463a4a4