Allow to return values from dolist, dotimes and do-symbols

[Affonso-Gui]

Partially solves #241

It is not as complete as the gist solution on the discussion, but is done with minimal changes and is sufficient for cases when the return value is omitted.

(dolist (i (list 1 2 3))
  (print i)
  (return t))
1
t

Remains the same (= different from cl) when the return value is explicitly given:

(dolist (i (list 1 2 3) 10)
  (print i)
  (return t))
1
10

[k-okada]

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