Allow to re-define `let`'s type in neighbour/inner/parent contexts with same name

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Example of problem:

let(stuff :: String) { "stuff" }

context "some" do
  let(stuff :: Int32) { 42 }
  # => Can't cast 42 to String
end

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This happens, since let defines stuff macro as macro stuff (no args). So first defined will always be used, since they have one scope.

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Have no idea how to fix it at this point. At least without any sort of scoping.

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Maybe it is possible RFC to crystal to be able to define scopes for macros somehow:

macro stuff
  "stuff"
end
stuff # => "stuff"

macro_scope
  # .. there is no `stuff` macro here .. so:
  macro stuff
    42
  end
  stuff # => 42
end

[waterlink]

Maybe it is not actually a problem, just use different names for different types in specs?

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When second let is of the same type, everything works correctly as expected.