Plusminus 0.3.0

- Added syntax to clear a defined variable:

      a, b = 1, 2
      c = a + b    -> 3
      a =          -> clears variable a
      c = a + b    -> NameError

- Added support for nested sets, and better set display format.

      {{1, 2}, 99, 100}
      {99, 'z', 'a'} ∪ {'a', 't', 100} -> {99, 100, 'a', 't', 'z'}

- Set literals can be used as function arguments, if supported
  by the function.

      max({1, 2, 4})  -> 4
      min({1, 2, 4})  -> 1
      sin({1, 2, 4})  -> TypeError

- Added dict-like access API to set and get variables defined within
  a parser:

      for x in range(10):
          parser['x'] = x
          parser.evaluate("y = x * x")
          print(parser['y'])
          del parser['y']

- plusminus has been hardened against some possible attacks, using
  deep expression nesting or formula references:

  - To guard against expressions that are too deeply nested, a
    customizable maximum_expression_depth attribute has been added
    to parsers. Parsers customized with additional operators may need
    to limit the allowed depth. The default maximum depth is 6
    (reduced from 10 in 0.2.0).

        ((((((0)))))) -> 0
        (((((((0))))))) -> OverflowError: too deeply nested

    There is also a maximum_set_depth attribute for nested sets,
    also defaults to 6.

  - A similar performance issue can be raised if a formula chains
    to another formula to another formula, etc. too deeply.

        a @= b + b
        b @= c + c
        c @= d + d
        ...
        m @= n + n -> OverflowError: function variables nested too deeply

    A customizable parser attribute maximum_formula_depth will limit the number
    of formula indirections. The default value is 12.

  - An attack may try to define too many variables and crash an application
    by consuming excessive memory. A value to limit the number of variables and
    their respective memory usage was previously hard-coded. These are now
    part of the public API for parsers: max_number_of_vars (default = 1000)
    and max_var_memory (default = 10MB).