Delegate methods in Trait

Delegating #instance_methods and #send to the trait's anon module allows
us to not switch on kind in the Composer.
Rename #trait! to #methods!

Since we're asking for the trait's methods, not itself, this is more
descriptive.
Move composition specs into spec for Composer

This is actually what we're describing, even though the tests are more
end-to-end rather than unit.