Improve cond<->if

[nbfalcon]

• Unify lispy-to-cond and lispy-to-ifs into a single, self-reversing function lispy-cond<->if-dwim
• Bugfix: handle empty conds without maximum recursion depth errors (lispy--to-ifs has no base case for that)
• Bugfix: lispy-cond<->-if-dwim: don't error if the result is a single else-tail symbol (cond (t t)) -> t and user-error, because lispy-different fails (ignore user-error)
• Bugfix: don't silently swallow else-tail-tails: previously (cond (t X XS...)) -> X, now (progn X XS...)
• Feature: cond now supports bracketed conditions: (cond [t X XS...]) (Racket convention)
• Feature: cond now supports :else as an else tail (used in the dash library) and also a bare else in scheme-mode:

  (cond (else X XS...)) ; -> (progn X XS...)

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[coveralls]

Coverage decreased (-2.6%) to 60.211% when pulling 7d9c9b2b0962d8382b62183a257bcc111319a318 on nbfalcon:feature/improve-cond<->if into 1ad128be0afc04b58967c1158439d99931becef4 on abo-abo:master.