associated type normalization syntax in surface rust differs from real rust

[nikomatsakis]

The rust layer permits things like

where
    [(< T as Iterator[] > :: Item[] == u32)]

but this is not really what we support in regular Rust (T: Iterator<Item = u32>). There's been talk of adding == syntax to regular Rust (see rust-lang/rust#20041) but it's not supported now. Meanwhile, the syntax we do support is more general, since one can do things like T: Foo<Bar = u32, Baz = i32>.

Also, in regular Rust, we lower T: Foo<Bar = u32> to both T: Foo and <T as Foo>::Bar = u32.