Fix some grammar issues.

[ehuss]

• trait was missing bounds.
  trait T: Clone {}
• Type for const in trait is not optional.

  trait T {
      const C = 1;  // INVALID
      const C2: i32 = 1;
  }

• Associated type in trait can have a bare ::
  trait T { type X:; }
• Visibility is not needed on UseDeclaration, it is part of Item.
• : in generic lifetime param was missing parenthesis grouping to make it optional.
  fn f<'a>() {}

[matthewjasper]

Changes look good, there's a couple more things that could be fixed in these files if you have time.

[matthewjasper]

Thanks!