String concat contains dummy comparision.  #158

# strings length is a ( 100 + rand( 100 ) ), i.e. short strings max difference around double 

 String#append:   439150.7 i/s
            String#+:   419672.1 i/s - same-ish: difference falls within error
  "#{'foo'}#{'bar'}":   365799.6 i/s - 1.20x  slower
       String#concat:   353912.5 i/s - 1.24x  slower

 # strings length is a  (1000 + rand( 1000) ) , i.e. longer strings max difference around double

            String#+:   298633.5 i/s
   String#append:   284566.8 i/s - same-ish: difference falls within error
  "#{'foo'}#{'bar'}":   267548.4 i/s - 1.12x  slower
    String#concat:   215419.4 i/s - 1.39x  slower

# string length is a ( 100 + rand(1000) ), i.e. highly variable length
             String#+:   388205.2 i/s
   String#append:   357669.3 i/s - 1.09x  slower
  "#{'foo'}#{'bar'}":   331617.7 i/s - 1.17x  slower
    String#concat:   286717.8 i/s - 1.35x  slower

# string length is a 1 + rand 1000, i.e. random string
             String#+:   385180.8 i/s
   String#append:   373926.8 i/s - same-ish: difference falls within error
  "#{'foo'}#{'bar'}":   341496.6 i/s - 1.13x  slower
   String#concat:   297988.1 i/s - 1.29x  slower

# This is completely oposite to
         "foo" "bar":  5369594.5 i/s
 "#{'foo'}#{'bar'}":  5181745.7 i/s - same-ish: difference falls within error
   String#append:  3075719.2 i/s - 1.75x slower
    String#concat:  3016703.5 i/s - 1.78x slower
            String#+:  2977282.7 i/s - 1.80x slower