Remove commas from trait inheritance list

[brson]

The trait inheritance list is a list of type bounds, but for traits. These two constructs mean very similar, if not the same, things, 'in order for a type to be Foo it must be Bar'.

fn f<Foo: Bar>() { }

trait Foo: Bar { }

but the syntaxes suggest dissimilarity with more than one bound:

fn f<Foo: Bar Baz>() { }

trait Foo: Bar, Baz { }

Using space instead of comma in the trait declaration would be more consistent and looks fine.

fn f<Foo: Bar Baz>() { }

trait Foo: Bar Baz { }

[catamorphism]

Sounds good to me.

[brson]

Fixed by 10c9d79