Speed up is_a_spec

[farleyknight]

Speed up is_a_spec by about 6 seconds on my work laptop.

farleyknight:~/dev/crystal/crystal$ rspec spec/codegen/is_a_spec.rb 
Run options: exclude {:primitives=>true, :integration=>true}
.....................

Finished in 4.01 seconds
21 examples, 0 failures
farleyknight:~/dev/crystal/crystal$ rspec spec/codegen/is_a_spec.rb 
Run options: exclude {:primitives=>true, :integration=>true}
.....................

Finished in 10.62 seconds
21 examples, 0 failures

[coveralls]

Coverage remained the same when pulling e0479dd on farleyknight:speed_up_is_a_spec into 26b1236 on manastech:master.

[asterite]

Sorry, but no. Each spec should have a single Program. If you reuse a Program across many runs than all definitions conflict: you reopen classes, bind variables to new types, etc.

In this case it might succeed, but just because nothing is overwritten in each separate spec. But what if we add another spec that conflicts with the others?