[PATCH] Module#local_constants is slow as dirt

This misses a case

module Foo
  A = 1
end

class Bar
  include Foo
  A = 'b'
end

Bar.local_constants should return A? so Bar::A will be reloaded, but this patch will return A? since it can't distinguish the mixed-in Foo::A from Bar::A.

Also, caching the local constants can't work since they're reloaded per request in development. In production, reloading and hence this method aren't used.

Ironing out these hotspots is welcome but I haven't figured out how to crack this one without doing a bunch of const_get to compare the objects the constants reference.