auto merge of #5556 : nikomatsakis/rust/issues-4904-and-4855, r=nikomatsakis

Two small fixes. 

r? @catamorphism
Rip out old code that still structured method calls as a
expr_call(expr_field(...)) rather than an expr_method_call.
There is probably more such code in trans that should be removed.
Add various debug statements to trans that I used to help track down the
problem and which seem like they could be useful in the future.
Stop writing directly to the final type/method/vtable sidetables from astconv
and from typeck, which is verboten.  We are supposed to write inference results
into the FnCtxt and then these get copied over in writeback.  Add assertions
that no inference by-products are added to this table.

Fixes #3888
Fixes #4036
Fixes #4492