Plusminus 0.2.0

0.2.0 -

- Added set notation and arithmetic:
      {1, 2, 3} is the set of the integers 1, 2, and 3
      {} is the empty set
      a ∩ b is the intersection of sets a and b
      a ∪ b is the union of sets a and b
      a ∈ b a is an element of b (can also be written 'a in b')
      a ∉ b a is not an element of b (can also be written 'a not in b')

- Replaced "between", "within", and "in range from-to" operators
  to single "in" operator, taking an argument of the form:
      [1, 10] between 1 and 10 (including the values 1 and 10)
      [1, 10) between 1 and 10 (excluding the value 10)
      (1, 10] between 1 and 10 (excluding the value 1)
      (1, 10) between 1 and 10 (excluding the values 1 and 10)

- Custom functions can now take variable numbers of arguments,
  using ... for the arity. For example, here is a variant of
  hypot computing an n-dimensional distance:

      self.add_function("nhypot", ..., lambda *seq: sum(safe_pow(i, 2) for i in seq)**0.5)

- Updated the values returned from evaluating an assignment
  expression. If a single value is assigned, then a single
  value is returned. If multiple values are assigned, a tuple
  of the values is returned. (Previously, the underlying
  list of values was returned.)

- Guard against overly-nested ()'s (10 levels is the max).

- Changed signature of safe_pow, now takes multiple operands
  instead of a tuple of operands.

- New unit tests, thanks toonarmycaptain!