Optimize Schema.set_class

[casperisfine]

constants.include? allocate an array every time and does an O(N) search.

const_defined? saves that allocation and does an O(1) lookup.

Additionally it's more correct since it will return true for private constants while Module#constants won't include private constants.

For context one of our app spend 10-15% of boot time in there:

==================================
  Mode: wall(1000)
  Samples: 17329 (4.89% miss rate)
  GC: 3219 (18.58%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
      2657  (15.3%)        2657  (15.3%)     (marking)
      2524  (14.6%)        2524  (14.6%)     Module#constants

==================================
  Mode: cpu(1000)
  Samples: 20400 (4.93% miss rate)
  GC: 4513 (22.12%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
      3856  (18.9%)        3856  (18.9%)     (marking)
      2569  (12.6%)        2569  (12.6%)     Module#constants