Stop depending on the real stdlib for our specs

[bew]

Here is an exemple of spec output, where all failures are because the stdlib has new functions than when the specs where written..

 ~open/scry  % cr spec --progress
...............................................FFFFFFFFFFFF.............FF.................................

Failures:

  1) Scry::CompletionProvider method completion int32 completes "a = 1\n                        a."
     Failure/Error: it_completes "a = #{i}

       Expected: ["-", "clone", "popcount"]
            got: ["-", "clone", "leading_zeros_count", "popcount", "trailing_zeros_count"]

     # spec/scry/completion_provider_spec.cr:70

..... many more like this

I think that the completion specs should be rewritten to not depend on external contexts (the current stdlib)

Note: in #158 I worked around this issue by making a custom spec matcher that works when the completions items have AT LEAST the given methods. I don't think it is a good permanent solution though..