Refactor the self-info so that the def-id is carried in ty_self()
and the fn_ctxt doesn't need any self_info field at all.

Step towards fixing `fn(&self)` (cc #4846) to have a distinct
lifetime.
Apply explicit self transformation before we enter check_fn
auto merge of #5435 : nikomatsakis/rust/issue-4846-refactor-self_info, r=nikomatsakis

Refactor the self-info so that the def-id is carried in ty_self()and the fn_ctxt doesn't need any self_info field at all. Pull out explicit self transformation into `check_method`. Step towards fixing `fn(&self)` to have a distinct lifetime. (cc #4846) 

r? @catamorphism