Add deduplication to Errno objects

[herwinw]

This makes Errno spec-compliant

[herwinw]

Since the error messages appear to be removed in the output: it now fails with this:

bin/natalie -I test/support test/natalie/gc_test.rb
.F

Failed specs:
GC
  .enable and .disable
    control automatic garbage collection
      false should be == to true
      (test/natalie/gc_test.rb:44:in 'block in block in block in block')

1 spec(s) passed.
1 spec(s) failed.
0 spec(s) errored.
rake aborted!

So it looks like somehow this change has disabled the garbage collector