process trait/impl items directly from the visitor callback

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)