Per the straw poll in #908, as an alternative to #1147.
Now you must use `True`, `False`, and `None`, as in Python. Or just assign `true` to `True`, etc.; the old synonyms aren't reserved words anymore.
This changes let to use a flat list of symbol-value pairs instead of a
vector of vectors. One side effect is that (let [[a 1] z]) is not
expressible now, and one will explicitly need to set a nil value for z,
such as: (let [a 1 z nil]).
Closes#713.
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Re-indent `fib` examples to match that in original blog post - "Indentation shall be two spaces, except where matching the indentation of the previous line"
