I'm not quite sure what this comment means... is "true if `ty` has finite size, without any unboxed structural recursion" a reasonable interpretation?

(Also three minor points: the return value is no longer `bool`, and if you use `///` rather than `//`, this will be a doc comment and will be displayed in [the docs](http://static.rust-lang.org/doc/master/rustc/middle/ty/index.html) and similarly for `Repr`, and, lastly, could we call `Repr` something more descriptive, like, maybe `Representability`, since `Repr` is a rather short and generic name.)