Things we want to support:

- basic applying of traits to methods:
    command foo => $code;
  should install $code as 'foo' in the current class, with the command trait
  applied
- setting attributes in the traits that we apply:
    command foo => $code, formatter => $format;
  should install $code as 'foo', and set the formatter attribute to $format on
  the method metaobject
- passing in things other than coderefs
    alias foo => 'bar';
  should be able to look 'bar' up in the current package, and install foo as an
  alias to it (potentially with a trait applied)
- applying traits to existing methods
    command 'foo';
  should find an existing 'foo' method in the current class, and apply the
  command trait to it