Fix limited collection vs singleton subtype.

Given that <type> can not be a subtype of <singleton> and the only
thing defined as a possible subtype of a singleton is another
identical singleton, it makes sense to have this also result
in a result of ``#f``.

This fixes this runtime warning:

  Warning: Method specificity of unorderable methods
      {<pair>:
        {<simple-method>: ??? (<&type>, <&singleton>)},
        {<simple-method>: ??? (<&limited-collection-type>, <&type>)}}
  applying
      {<sealed-generic-function>: ^subtype?}
  to
      {<simple-object-vector>: {<&limited-sequence-type>}, {<&singleton>}}
  was determined with arbitrary and capricious rules.