normalize trait-ref in context of impl
normalize trait-ref in context of impl

The `specializes()` function was trying to normalize the impl trait in
an empty environment. This could lead to inexplicable failures.

graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust