v0.4.0

• Add ?Sized relaxation to impl header whenever possible #54
• Check for Self trait bounds on methods and add those bounds to impl header #54 (and fix in #67)
• Make sure that auto_impl custom attributes are only applied to methods #59

Internals

• Update syn, quote and proc-macro2 to 1.0 #57
• Use trybuild and move to proc-macro-error #60