Remove old syntax for structs implementing a trait

[erickt]

The rust grammar still allows for this old-style struct implementing a trait:

trait T { ... }
struct S: T { ... }

Using this form is no longer safe though, as I experienced in #3276. The new min/max style works though:

trait T { }
struct S { ... }
impl S: T { ... }

[Blei]

This seems to have been fixed in the mean time. I'm getting errors like:

failstruct.rs:3:22: 3:25 error: obsolete syntax: class traits
failstruct.rs:3 struct Deserializer : std::serialization::deserializer{
                                      ^~~
note: implemented traits are specified on the impl, as in `impl foo : bar {`
failstruct.rs:5:4: 5:7 error: obsolete syntax: struct constructor
failstruct.rs:5     new() { self.x = (); }

...

When trying to compile the example in the linked bug report.

[catamorphism]

Yes, this is done.