Remove ret_style and instead check whether fn return type is bot

[nikomatsakis]

On my branch for fn inference I encountered #3681. Here is a fix.

r? @catamorphism

[catamorphism]

r+ - I was concerned that changing this would allow the _|_ type to appear in places where it shouldn't, but I guess since you haven't changed the syntax (and thus haven't added syntax for _|_ as a first-class type), that's not possible.

[nikomatsakis]

pushed 9f7dc1c