177 bug incorrectly allowing variable to be callable

[mrodz]

• Added is operator (works on Object, Module, Vec, and falls back onto ==)

  a = [1, 2, 3]
  b = [1, 2, 3]
  c = a
  
  print a == b # true
  print a is b # false
  print a == c # true
  print a is c # true

• Fixed bug with Optional and ClassSelf not being handled properly by the typechecker in lists (#177)
• Fixed runtime crash where a Class instance was passed to a standard call instruction because the compiler thought it was the class' constructor, and marked it as callable.

  class A {}
  a = A()
  a() # << HERE -- fixed